Is territorial expansion a mechanism for crossmodal plasticity?

Crossmodal plasticity is the phenomenon whereby, following sensory damage or deprivation, the lost sensory function of a brain region is replaced by one of the remaining senses. One of several proposed mechanisms for this phenomenon involves the expansion of a more active brain region at the expense of another whose sensory inputs have been damaged or lost. This territorial expansion hypothesis was examined in the present study. The cat ectosylvian visual area (AEV) borders the auditory field of the anterior ectosylvian sulcus (FAES), which becomes visually reorganized in the early deaf. If this crossmodal effect in the FAES is due to the expansion of the adjoining AEV into the territory of the FAES after hearing loss, then the reorganized FAES should exhibit connectional features characteristic of the AEV. However, tracer injections revealed significantly different patterns of cortical connectivity between the AEV and the early deaf FAES, and substantial cytoarchitectonic and behavioral distinctions occur as well. Therefore, the crossmodal reorganization of the FAES cannot be mechanistically attributed to the expansion of the adjoining cortical territory of the AEV and an overwhelming number of recent studies now support unmasking of existing connections as the operative mechanism underlying crossmodal plasticity.

Behavioral Indices of Multisensory Integration: Orientation to Visual Cues is Affected by Auditory Stimuli

Physiological studies have demonstrated that inputs from different sensory modalities converge on, and are integrated by, individual superior colliculus neurons and that this integration is governed by specific spatial rules. The present experiments were an attempt to relate these neural processes to overt behavior by determining if behaviors believed to involve the circuitry of the superior colliculus would show similar multisensory dependencies and be subject to the same rules of integration. The neurophysiological-behavioral parallels proved to be striking. The effectiveness of a stimulus of one modality in eliciting attentive and orientation behaviors was dramatically affected by the presence of a stimulus from another modality in each of the three behavioral paradigms used here. Animals trained to approach a low intensity visual cue had their performance significantly enhanced when a brief, low intensity auditory stimulus was presented at the same location as the visual cue, but their performance was significantly depressed when the auditory stimulus was disparate to it. These effects were independent of the animals' experience with the modifying (i.e. auditory) stimulus and exceeded what might have been predicted statistically based on the animals' performance with each single-modality cue. The multiplicative nature of these multisensory interactions and their dependence on the relative positions and intensities of the two stimuli were all very similar to those observed physiologically for single cells. The few differences that were observed appeared to reflect the fact that understanding integration at the level of the single cell requires reference to the individual cell's multisensory receptive field properties, while at the behavioral level populations of receptive fields must be evaluated. These data illustrate that the rules governing multisensory integration at the level of the single cell also predict responses to these stimuli in the intact behaving organism.

Connectional parameters determine multisensory processing in a spiking network model of multisensory convergence

For the brain to synthesize information from different sensory modalities, connections from different sensory systems must converge onto individual neurons. However, despite being the definitive, first step in the multisensory process, little is known about multisensory convergence at the neuronal level. This lack of knowledge may be due to the difficulty for biological experiments to manipulate and test the connectional parameters that define convergence. Therefore, the present study used a computational network of spiking neurons to measure the influence of convergence from two separate projection areas on the responses of neurons in a convergent area. Systematic changes in the proportion of extrinsic projections, the proportion of intrinsic connections, or the amount of local inhibitory contacts affected the multisensory properties of neurons in the convergent area by influencing (1) the proportion of multisensory neurons generated, (2) the proportion of neurons that generate integrated multisensory responses, and (3) the magnitude of multisensory integration. These simulations provide insight into the connectional parameters of convergence that contribute to the generation of populations of multisensory neurons in different neural regions as well as indicate that the simple effect of multisensory convergence is sufficient to generate multisensory properties like those of biological multisensory neurons.

Somatosensory and multisensory properties of the medial bank of the ferret rostral suprasylvian sulcus

In ferret cortex, the rostral portion of the suprasylvian sulcus separates primary somatosensory cortex (SI) from the anterior auditory fields. The boundary of the SI extends to this sulcus, but the adjoining medial sulcal bank has been described as "unresponsive." Given its location between the representations of two different sensory modalities, it seems possible that the medial bank of the rostral suprasylvian sulcus (MRSS) might be multisensory in nature and contains neurons responsive to stimuli not examined by previous studies. The aim of this investigation was to determine if the MRSS contained tactile, auditory and/or multisensory neurons and to evaluate if its anatomical connections were consistent with these properties. The MRSS was found to be primarily responsive to low-threshold cutaneous stimulation, with regions of the head, neck and upper trunk represented somatotopically that were primarily connected with the SI face representation. Unlike the adjoining SI, the MRSS exhibited a different cytoarchitecture, its cutaneous representation was largely bilateral, and it contained a mixture of somatosensory, auditory and multisensory neurons. Despite the presence of multisensory neurons, however, auditory inputs exerted only modest effects on tactile processing in MRSS neurons and showed no influence on the averaged population response. These results identify the MRSS as a distinct, higher order somatosensory region as well as demonstrate that an area containing multisensory neurons may not necessarily exhibit activity indicative of multisensory processing at the population level.

Stimulus intensity modifies saccadic reaction time and visual response latency in the superior colliculus

Performance in a reaction time task can be strongly influenced by the physical properties of the stimuli used (e.g., position and intensity). The reduction in reaction time observed with higher-intensity visual stimuli has been suggested to arise from reduced processing time along the visual pathway. If this hypothesis is correct, activity should be registered in neurons sooner for higher-intensity stimuli. We evaluated this hypothesis by measuring the onset of neural activity in the intermediate layers of the superior colliculus while monkeys generated saccades to high or low-intensity visual stimuli. When stimulus intensity was high, the response onset latency was significantly reduced compared to low-intensity stimuli. As a result, the minimum time for visually triggered saccades was reduced, accounting for the shorter saccadic reaction times (SRTs) observed following high-intensity stimuli. Our results establish a link between changes in neural activity related to stimulus intensity and changes to SRTs, which supports the hypothesis that shorter SRTs with higher-intensity stimuli are due to reduced processing time.

Spatial distribution of functional superficial-deep connections in the adult ferret superior colliculus

Numerous studies have identified connections between the superficial visual and deeper multisensory layers of the superior colliculus (SC), but the functional distribution of the superficial-deep projection has not been mapped. This question was assessed in the present study using extracellular electrophysiological stimulation and recording techniques. In vitro slices from adult ferrets were used to functionally map the rostro-caudal, medio-lateral, and dorso-ventral distribution of these superficial-deep connections. For each coronal (n=6) or parasagittal (n=10) slice, single and multi-unit responses to electrical stimulation of a point in the superficial layers were systematically recorded at different locations along a grid (approximately 300 microm intervals) across the slice. Recording sites with similar activation thresholds were grouped on the histological reconstruction of each slice to plot the functional access of superficial stimulation site to the deeper layers. Low intensity stimulation (defined as a current threshold < or =75 microA) activated areas of the subjacent intermediate layers in most cases (75%; 12/16). Higher intensity stimuli (> 75-600 microA) accessed larger areas which, in 50% of the slices, extended into the deepest layers of the SC. However, regardless of the rostro-caudal or medio-lateral position of the superficial layer stimulation site, the proportion of the deeper layers activated remained remarkably constant, although the volume of activated deep layer tissue was shifted in each case toward the central regions of the SC. This last observation argues against the precise alignment of the superficial and deep layer visual maps, suggesting instead that the arrangement of the superficial layer projection may more closely relate to the organization of deep layer auditory and/or somatosensory representations.

Engagement of visual fixation suppresses sensory responsiveness and multisensory integration in the primate superior colliculus

Neurons in the intermediate and deep layers of the superior colliculus (SC) often exhibit sensory-related activity in addition to discharging for saccadic eye movements. These two patterns of activity can combine so that modifications of the sensory response can lead to changes in orienting behaviour. Can behavioural factors, however, influence sensory activity? In this study of rhesus monkeys, we isolate one behavioural factor, the state of visual fixation, and examine its influences on sensory processing and multisensory integration in the primate SC. Two interleaved fixation conditions were used: a FIX condition requiring exogenous fixation of a visible fixation point; and a FIX-BLINK condition, requiring endogenous fixation in the absence of a visible fixation point. Neurons of the SC were influenced by fixation state, exhibiting both lower levels of sensory activity and reduced multisensory interactions when fixation was exogenously engaged on a visible fixation point. These results are consistent with active visual fixation suppressing responses to extraneous stimuli, and thus demonstrate that sensory processing and multisensory responses in the SC are not dependent solely on the physical properties of the sensory environment, but are also dynamically influenced by the behavioural state of the animal.

A comparison of the distribution of GABA-ergic neurons in cortices representing different sensory modalities

It is well known that sensory receptive field properties are shaped by inhibitory processes. Given the physiological and perceptual distinctions among the different sensory modalities, it might be expected that the contribution of GABA-ergic inhibition to the process would vary from area to area, depending on the sensory modality represented. Furthermore, as receptive field properties become progressively more complex at higher cortical levels, differences in the inhibitory contributions to these computations would be reflected in differences in GABA-ergic neuronal distribution. These possibilities were examined in the cortices surrounding the cat Anterior Ectosylvian Sulcus (AES) which contains higher order visual (AEV), somatosensory (SIV) and auditory (Field AES) representations, and is located between the lower-level primary (AI) and secondary auditory (AII) and somatosensory (SII) areas. Using standard immunocytochemical and light-microscopic techniques, the distribution of GABA-ergic neurons (and their co-localized calcium-binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV)) was determined for each area. When normalized for differences in cortical thickness, the depth distribution of each of the immunopositive types was plotted. These data confirmed that there were striking differences in the distribution of GABA-, CB-, CR- and PV-positive neurons. However, the laminar organization for a given marker was remarkably similar for the different subregions, irrespective of modality or hierarchical level. These data indicate that, instead of underlying processing differences among different sensory and hierarchical representations, the distribution of GABA-ergic inhibitory neurons reveals common organizational features across sensory cortex.

Organization of the neurons of origin of the descending pathways from the ferret superior colliculus

The superior colliculus (SC), through its descending projections to the brainstem and spinal cord, is involved in initiating sensory-driven orienting behaviors. Ferrets are carnivores that hunt both above and below ground using visual (and auditory) cues in the daylight but non-visual cues in darkness and in subterranean environments. The present investigation sought to determine whether the ferret SC shows organizational features similar to those found in other visually dominant animals (e.g. cats), or whether characteristics of colliculi from non-visually dominant animals (e.g. rodents) prevail. Injection of retrograde tracer into the identified targets of the colliculus (cervical spinal cord, the contralateral pontomedullary reticular formation, or the ipsilateral pontine reticular formation) labeled tectospinal, crossed tectoreticular, and ipsilateral tectoreticular neurons, respectively, within the adult ferret SC. Labeled tectospinal and crossed tectoreticular neurons were far outnumbered by neurons with ipsilateral reticular projections. Like those of their visually dominant relatives, ferret tectospinal neurons were well represented throughout the anterior-posterior extent of the SC and crossed tectoreticular neurons tended to be distributed more broadly across the intermediate gray layer than those of rodents. Thus, even though ferrets perform well as subterranean predators where non-visual cues initiate orienting behaviors, these anatomical characteristics indicate that their colliculi are organized similar to that of their visually dominant, carnivorous relatives.

The influence of stimulus properties on multisensory processing in the awake primate superior colliculus

Multisensory integration is a process whereby information converges from different sensory modalities to produce a response that is different from that elicited by the individual modalities presented alone. A neural basis for multisensory integration has been identified within a variety of brain regions, but the most thoroughly examined model has been that of the superior colliculus (SC). Multisensory processing in the SC of anaesthetized animals has been shown to be dependent on the physical parameters of the individual stimuli presented (e.g., intensity, direction, velocity) as well as their spatial relationship. However, it is unknown whether these stimulus features are important, or evident, in the awake behaving animal. To address this question, we evaluated the influence of physical properties of sensory stimuli (visual intensity, direction, and velocity; auditory intensity and location) on sensory activity and multisensory integration of SC neurons in awake, behaving primates. Monkeys were trained to fixate a central visual fixation point while visual and/or auditory stimuli were presented in the periphery. Visual stimuli were always presented within the contralateral receptive field of the neuron whereas auditory stimuli were presented at either ipsi- or contralateral locations. Many of the SC neurons responsive to these sensory stimuli (n = 66/84; 76%) had stronger responses when the visual and auditory stimuli were combined at contralateral locations than when the auditory stimulus was located on the ipsilateral side. This trend was significant across the population of auditory-responsive neurons. In addition, some SC neurons (n = 31) were presented a battery of tests in which the quality of one stimulus of a pair was systematically manipulated. A small proportion of these neurons (n = 8/31; 26%) showed preferential responses to stimuli with specific physical properties, and these preferences were not significantly altered when multisensory stimulus combinations were presented. These data demonstrate that multisensory processing in the awake behaving primate is influenced by the spatial congruency of the stimuli as well as their individual physical properties.

Responses to innocuous, but not noxious, somatosensory stimulation by neurons in the ferret superior colliculus

The intermediate and deep layers of the superior colliculus (SC) are known for their role in initiating orienting behaviors. To direct these orienting functions, the SC of some animals (e.g., primates, carnivores) is dominated by inputs from the distance senses (vision, audition). In contrast, the rodent SC relies more heavily on non-visual inputs, such as touch and nociception, possibly as an adaptive response to the proximity of dangers encountered during their somatosensory-dominant search behaviors. The ferret (a carnivore) seems to employ strategies of both groups: above ground they use visual/auditory cues, but during subterranean hunting ferrets must rely on non-visual signals to direct orienting. Therefore, the present experiments sought to determine whether the sensory inputs to the ferret SC reveal adaptations common to functioning in both environments. The results showed that the ferret SC is dominated (63%; 181/286) by visual/auditory inputs (like the cat), rather than by somatosensory inputs (as found in rodents). Furthermore, tactile responses were driven primarily from hair-receptors (like cats), not from the vibrissae (as in rodents). Additionally, while a majority of collicular neurons in rodents respond to brief noxious stimulation, no such neurons were encountered in the ferret SC. A small proportion (4%; 13/286) of the ferret SC neurons were responsive to long-duration (>5 s) noxious stimulation, but further tests could not establish these responses as nociceptive. Collectively, these data indicate that the ferret SC is best adapted for the animal's visuallacoustically guided activities and most closely resembles the SC of its phylogenetic relative, the cat.

The frontal eye fields target multisensory neurons in cat superior colliculus

While sensory corticotectal connections have received considerable attention, relatively little is known about the nature of superior colliculus neurons that receive input from the cortical frontal eye fields. The present experiments used microstimulation of indwelling electrodes in the frontal eye fields and single-unit recording in the superior colliculus to demonstrate that frontal afferents preferentially terminate on multisensory neurons in the colliculus. Furthermore, the medial and lateral subdivisions of the cat frontal eye fields access physiologically distinct populations of multisensory collicular neurons. Specifically, the medial subdivision preferentially activates neurons with visual and auditory sensory responses located medial within the colliculus, while the lateral subdivision preferentially activates collicular neurons with visual and somatosensory responses found more laterally. These data support reports distinguishing the medial and lateral subdivisions of the frontal eye fields in the cat and suggest that signals from each may route separately through the colliculus to induce or coordinate different components of gaze control.

Extraocular motor unit and whole-muscle responses in the lateral rectus muscle of the squirrel monkey

Because primate studies provide data for the current experimental models of the human oculomotor system, we investigated the relationship of lateral rectus muscle motoneuron firing to muscle unit contractile characteristics in the squirrel monkey. Also examined was the correlation of whole-muscle contractile force with the degree of evoked eye displacement. A force transducer was used to record lateral rectus whole-muscle or muscle unit contraction in response to abducens whole-nerve stimulation or stimulation of single abducens motoneurons or axons. Horizontal eye displacement was recorded using a magnetic search coil. (1) Motor units could be categorized based on contraction speed (fusion frequency) and fatigue. (2) The kt value (change in motoneuronal firing necessary to increase motor unit force by 1.0 mg) of the units correlated with maximum tetanic tension. (3) There was some tendency for maximum tetanic tension of this unit population to separate into three groups. (4) At a constant frequency of 100 Hz, 95% of the motor units demonstrated significantly different force levels dependent on immediately previous stimulation history (hysteresis). (5) A mean force change of 0.32 gm/ degrees and a mean frequency change of 4.7 Hz/ degrees of eye displacement were observed in response to whole-nerve stimulation. These quantitative data provide the first contractile measures of primate extraocular motor units. Models of eye movement dynamics may need to consider the nonlinear transformations observed between stimulation rate and muscle tension as well as the probability that as few as two to three motor units can deviate the eye 1 degrees.

Suppression of NMDA receptor function using antisense DNA block ocular dominance plasticity while preserving visual responses

Pioneering work has shown that pharmacological blockade of the N-methyl-D-aspartate (NMDA) receptor channel reduces ocular dominance plasticity. However, the results also show that doses of NMDA receptor antagonists that have an effect on ocular dominance plasticity profoundly reduce sensory responses and disrupt stimulus selectivity of cortical cells. It is, therefore, not possible to determine whether effects of NMDA receptor blockade on visual plasticity result from a specific role of NMDA receptors or from the reduction in sensory response. We have used an alternate approach to examine this question. We performed knockdown experiments using antisense oligodeoxynucleotides (ODNs) complementary to mRNA coding the NR1 subunit of the NMDA receptor. After 5 days of antisense, but not sense, ODN treatment NMDA receptor-mediated synaptic transmission was reduced markedly relative to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor response, as indicated by whole cell patch-clamp recordings in the cortical slice preparation. This suppression of NMDA receptor-mediated currents was due to a selective reduction in the NR1 protein near the injection site relative to the untreated hemisphere in the same animal, as indicated by immunocytochemistry and Western blotting. In contrast, AMPA receptors were not affected by the antisense ODN treatment indicating specificity of effects. Another major effect of this treatment was to decrease ocular dominance plasticity. Ferrets that were monocularly deprived 1 wk during the antisense ODN treatment had ocular dominance histograms similar to those found in untreated, nondeprived animals. In contrast, ferrets treated with sense ODN and monocularly deprived had ocular dominance histograms resembling those of untreated, monocularly deprived animals. The effects on ocular dominance plasticity did not result from a disruption of sensory responses because maximum responses as well as orientation and direction selectivity of cortical cells were not affected by the treatment. In conclusion, the present results show that antisense techniques can accomplish more selective manipulations of cortical function than is possible with traditional pharmacological agents. Use of this approach also provides unambiguous evidence for a specific role of NMDA receptors in visual plasticity.

Multisensory integration in the superior colliculus of the alert cat

The modality convergence patterns, sensory response properties, and principles governing multisensory integration in the superior colliculus (SC) of the alert cat were found to have fundamental similarities to those in anesthetized animals. Of particular interest was the observation that, in a manner indistinguishable from the anesthetized animal, combinations of two different sensory stimuli significantly enhanced the responses of SC neurons above those evoked by either unimodal stimulus. These observations are consistent with the speculation that there is a functional link among multisensory integration in individual SC neurons and cross-modality attentive and orientation behaviors.

Intrinsic circuitry of the superior colliculus: pharmacophysiological identification of horizontally oriented inhibitory interneurons

Much of what is known about the organization of the superior colliculus is based on the arrangement of its external connections. Consequently, there is little information regarding pathways that remain intrinsic to it, even though recent data suggest that a horizontally oriented local circuit may mediate the functional reciprocity among fixation and saccade-related neurons. Therefore, the present experiments sought physiological evidence for neurons intrinsic to the superior colliculus that might participate in a horizontally oriented local circuit. Parasagittal slices of the ferret superior colliculus were prepared for in vitro recording, and 125 intermediate/deep layer neurons were examined in response to electrical stimulation rostral or caudal to the recording site. A substantial proportion (37%) of neurons responded with a prolonged period (means = 59.3 +/- 30 ms) of poststimulus suppression of spontaneous action potential activity. Of the suppressed neurons, most (53%) were disinhibited when the excitatory amino acid receptor antagonists D-2-amino-5-phosphonovaleric acid (D-APV) and 6-nitro-7 sulphamoylbeno[f]-quinoxaline-2,3-dione (NBQX) were administered, indicating that excitatory input to inhibitory interneurons was blocked. Of the neurons that received inputs from inhibitory interneurons, all had their suppressive responses decreased or eliminated by the gamma-aminobutyric acid antagonist, bicuculline. Finally, severing the superficial layers from the slice had no effect on intermediate layer responses to intrinsic stimulation. These data provide physiological evidence for the presence of horizontally oriented inhibitory interneurons in the superior colliculus. Furthermore, these findings are consistent with the hypothesis that an intrinsic circuit, routed through interneurons, might account for the reciprocal inhibition observed among fixation and saccade-related neurons.

The role of anterior ectosylvian cortex in cross-modality orientation and approach behavior

Physiological and behavioral studies in cat have shown that corticotectal influences play important roles in the information-processing capabilities of superior colliculus (SC) neurons. While corticotectal inputs from the anterior ectosylvian sulcus (AES) play a comparatively small role in the unimodal responses of SC neurons, they are particularly important in rendering these neurons capable of integrating information from different sensory modalities (e.g., visual and auditory). The present experiments examined the behavioral consequences of depriving SC neurons of AES inputs, and thereby compromising their ability to integrate visual and auditory information. Selective deactivation of a variety of other cortical areas (posterolateral lateral suprasylvian cortex, PLLS; primary auditory cortex, AI; or primary visual cortex, 17/18) served as controls. Cats were trained in a perimetry device to ignore a brief, low-intensity auditory stimulus but to orient toward and approach a nearthreshold visual stimulus (a light-emitting diode, LED) to obtain food. The LED was presented at different eccentricities either alone (unimodal) or combined with the auditory stimulus (multisensory). Subsequent deactivation of the AES, with focal injections of a local anesthetic, had no effect on responses to unimodal cues regardless of their location. However, it profoundly, though reversibly, altered orientation and approach to multisensory stimuli in contralateral space. The characteristic enhancement of these responses observed when an auditory cue was presented in spatial correspondence with the visual stimulus was significantly degraded. Similarly, the inhibitory effect of a spatially disparate auditory cue was significantly ameliorated. The observed effects were specific to AES deactivation, as similar effects were not obtained with deactivation of PLLS, AI or 17/18, or saline injections into the AES. These observations are consistent with postulates that specific cortical-midbrain interactions are essential for the synthesis of multisensory information in the SC, and for the orientation and localization behaviors that depend on this synthesis.

Spatial determinants of multisensory integration in cat superior colliculus neurons

1. Although a representation of multisensory space is contained in the superior colliculus, little is known about the spatial requirements of multisensory stimuli that influence the activity of neurons here. Critical to this problem is an assessment of the registry of the different receptive fields within individual multisensory neurons. The present study was initiated to determine how closely the receptive fields of individual multisensory neurons are aligned, the physiological role of that alignment, and the possible functional consequences of inducing receptive-field misalignment. 2. Individual multisensory neurons in the superior colliculus of anesthetized, paralyzed cats were studied with the use of standard extracellular recording techniques. The receptive fields of multisensory neurons were large, as reported previously, but exhibited a surprisingly high degree of spatial coincidence. The average proportion of receptive-field overlap was 86% for the population of visual-auditory neurons sampled. 3. Because of this high degree of intersensory receptive-field correspondence, combined-modality stimuli that were coincident in space tended to fall within the excitatory regions of the receptive fields involved. The result was a significantly enhanced neuronal response in 88% of the multisensory neurons studied. If stimuli were spatially disparate, so that one fell outside its receptive field, either a decreased response occurred (56%), or no intersensory effect was apparent (44%). 4. The normal alignment of the different receptive fields of a multisensory neuron could be disrupted by passively displacing the eyes, pinnae, or limbs/body. In no case was a shift in location or size observed in a neuron's other receptive field(s) to compensate for this displacement. The physiological result of receptive-field misalignment was predictable and based on the location of the stimuli relative to the new positions of their respective receptive fields. Now, for example, one component of a spatially coincident pair of stimuli might fall outside its receptive field and inhibit the other's effects. 5. These data underscore the dependence of multisensory integrative responses on the relationship of the different stimuli to their corresponding receptive fields rather than to the spatial relationship of the stimuli to one another. Apparently, the alignment of different receptive fields for individual multisensory neurons ensures that responses to combinations of stimuli derived from the same event are integrated to increase the salience of that event. Therefore the maintenance of receptive-field alignment is critical for the appropriate integration of converging sensory signals and, ultimately, elicitation of adaptive behaviors.

Functional development of a central visual map in cat

1. The onset of visual activity in the superficial layers of the cat superior colliculus begins abruptly at about 6 days postnatal (DPN), just before natural eye opening. Despite the presence of many inactive sites at this time, the systematic nature of the superior colliculus visuotopy is already evident. The number of inactive sites across the horizontal dimension of the superficial layers decreases rapidly so that by 9-10 DPN most sites are visually responsive. 2. Initially, visual activity is restricted to the topmost portion of the superficial gray layer, where W-cell terminals predominate, but rapidly extends down to include Y-cell territory at 10 DPN. 3. In contrast to what might have been expected based on earlier behavioral observations, there was no physiological evidence for a central-to-peripheral gradient in the development of the superior colliculus visuotopy. Rather, the entire visual field is well represented long before the expression of any visually initiated behaviors. 4. In contrast to the rapidity of the appearance and organization of the visual representation in superficial layers, deep layers remain refractory to visual stimuli for weeks.

Converging influences from visual, auditory, and somatosensory cortices onto output neurons of the superior colliculus

1. Physiological methods were used to examine the pattern of inputs from different sensory cortices onto individual superior colliculus neurons. 2. Visual, auditory, and somatosensory influences from anterior ectosylvian sulcus (AES) and visual influences from lateral suprasylvian (LS) cortex were found to converge onto individual multisensory neurons in the cat superior colliculus. An excellent topographic relationship was found between the different sensory cortices and their target neurons in the superior colliculus. 3. Corticotectal inputs were derived solely from unimodal neurons. Multisensory neurons in AES and LS were not antidromically activated from the superior colliculus. 4. Orthodromic and antidromic latencies were consistent with monosynaptic corticotectal inputs arising from LS and the three subdivisions of AES (SIV, Field AES, and AEV). 5. Superior colliculus neurons that received convergent cortical inputs formed a principal component of the tecto-reticulospinal tract. Thus there appears to be extensive cortical control over the output neurons through which the superior colliculus mediates attentive and orientation behaviors. 6. Two other multisensory circuits were identified. A population of multisensory superior colliculus neurons was found, which neither received convergent cortical input nor projected into the tecto-reticulo-spinal tract. In addition, multisensory neurons in AES and LS proved to be independent of the superior colliculus (i.e., they were not corticotectal). While it is likely that these three distinct multisensory neural circuits have different functional roles, their constituent neurons appear to integrate their various sensory inputs in much the same way.

Integration of multiple sensory modalities in cat cortex

The results of this study show that the different receptive fields of multisensory neurons in the cortex of the cat anterior ectosylvian sulcus (AES) were in spatial register, and it is this register that determined the manner in which these neurons integrated multiple sensory stimuli. The functional properties of multisensory neurons in AES cortex bore fundamental similarities to those in other cortical and subcortical structures. These constancies in the principles of multisensory integration are likely to provide a basis for spatial coherence in information processing throughout the nervous system.

Visual, auditory and somatosensory convergence in output neurons of the cat superior colliculus: multisensory properties of the tecto-reticulo-spinal projection

A select population of superior colliculus (SC) neurons receives and integrates information from the visual, auditory and somatosensory systems. Determining which SC neurons comprise this population and where they send their multisensory messages is important in understanding the functional impact of the SC on attentive and orientation behavior. One of the major routes by which the SC influences these behaviors is the tecto-reticulo-spinal tract, a descending pathway that plays an integral role in the orientation of the eyes, ears and head. Of the 182 tecto-reticulo-spinal neurons (TRSNs) encountered in the present study, almost all (94%) responded to sensory stimuli and the overwhelming majority (84%) were multisensory. The present results demonstrate that the TRSN serves as an important link among the different sensory systems and provides a substrate through which they may gain access to the circuitry mediating orientation behavior.

Somatotopic component of the multisensory map in the deep laminae of the cat superior colliculus

The topographic organization of the somatosensory representation in the deep layers of the cat superior colliculus was reexamined using methods previously used to examine the visuotopy in these layers. This technique identified the distribution of neurons in the superior colliculus that represent a designated region of the body surface (i.e., a dermal image), as well as assessed the differential distribution of deep layer neurons representing different body regions (e.g., face, forelimb, hindlimb, etc.). When the area of densest representation within a dermal image was considered, a well-ordered somatotopy was evident that was similar to the one previously described (Stein et al., '76: J. Neurophysiol. 39:401-419). Each region of the body surface, however, was represented within a surprisingly broad area of the deep layers, which often had considerable overlap with the representations of adjacent body regions. This organization was similar to that of the deep layer visuotopy and emphasizes that the representation of a peripheral stimulus is accomplished by the simultaneous activation of a large population of deep layer neurons. Furthermore, an examination of the convergence patterns on somatosensory-responsive neurons demonstrated that the somatotopy was formed primarily by multisensory neurons. These data indicate that the somatosensory representation is best considered as a component of a comprehensive multisensory functional unit that plays a critical role in effecting behavioral responses to a wide variety of stimuli.

The visuotopic component of the multisensory map in the deep laminae of the cat superior colliculus

A well-defined map of visual space is located in the deep laminae of the cat superior colliculus. The horizontal meridian is oriented rostral-caudal, while the vertical meridian is oriented perpendicular to it in the rostral third of the structure. This map represents the entire contralateral visual field and extends approximately 40 degrees into ipsilateral visual space. Although the deep-laminae visuotopy is similar to that found in the superficial laminae of the same structure, the topographic register among these maps is most secure rostrally but becomes increasingly poorer at more caudal and lateral locations. The combination of 2 features distinguish the deep-layer visual representation from that found in the superficial laminae and in geniculocortical systems: (1) the constituent visual receptive fields are very large (mean diameter, 66.9 degrees), and (2) the majority (greater than 70%) of the neurons composing it receive nonvisual inputs. Because the visual receptive fields of visual-multisensory neurons are significantly larger than those of neighboring neurons that respond only to visual stimuli, far more visual-multisensory neurons are activated by any given visual stimulus. These data, when coupled with those from previous studies, suggest that, from a functional perspective, deep-laminae visual neurons form one component of an integrated multisensory map, and that their topographic organization is essential for the normal dynamics of multisensory integration.

Does an extraocular proprioceptive signal reach the superior colliculus?

1. The primary functions of the superior colliculus (SC) are thought to include both the spatial localization of sensory stimuli and the initiation of an orienting response. It has been hypothesized that, in cat, both of these SC functions may be influenced by feedback from the extraocular muscles. The present investigation was initiated to determine which SC cells receive this extraocular muscle feedback and how this feedback influences the discharge properties of SC cells and their ability to integrate input from other sensory modalities. These questions were addressed in cats prepared with various anesthetic agents. 2. During the course of these experiments it became apparent that responses of SC cells to extraocular muscle stimulation could be elicited only under very specific conditions, and these observations questioned the existence of functional extraocular inputs to SC cells. 3. Rotating the eye or stretching the extraocular muscles was never found to be effective in activating SC cells unless the drug chloralose was used in the experimental preparation. In these chloralose-anesthetized animals, responses to eye rotation or muscle stretch were long and variable in latency, the discharge did not reflect the metrics of the stimulus, and the velocity and amplitude thresholds of these cells usually exceeded the cat's oculomotor range. Responses usually consisted of one to three impulses and, in appropriate conditions, could be inhibited by responses to visual or auditory stimuli. 4. The origin of this SC response to eye rotation/extraocular-muscle stretch could not be localized to the extraocular muscles. Responses to passive stretch of extraocular muscles were not eliminated by anesthetizing the muscles with injections of lidocaine. Active contraction of the extraocular muscles induced by electrical stimulation of the oculomotor nerve was never observed to evoke SC responses. Furthermore, transection of the muscle nerves, which isolated the extraocular-muscle receptors from the CNS, did not affect the response initiated by stretching the extraocular muscles. However, in the absence of intact muscle nerves, pulling the periorbital tissue elicited responses very much like those produced by stretching the muscles in the intact preparation, suggesting the periorbita as the source of responses in intact preparations as well. 5. These data are not consistent with the hypothesis that feedback from extraocular-muscle receptors influences the activity of SC cells.

Auditory cortical projection from the anterior ectosylvian sulcus (Field AES) to the superior colliculus in the cat: an anatomical and electrophysiological study

Sensory activity in the deep layers of the superior colliculus (SC) is strongly influenced by descending cortical inputs. Elimination (permanent or reversible) of specific regions of visual or somatosensory cortex, known to have direct access to the SC, abolishes or dramatically reduces SC responses to stimuli from those modalities. While many SC neurons are also responsive to auditory cues, the origin of auditory corticotectal connections is not clear at present and their affect on activity in the SC is unknown. Therefore, the present study was undertaken to determine the origin, organization, and functional characteristics of auditory corticotectal projections. Of the auditory cortices (AI; AII; Fields A, P, and VP), only the auditory subregion of the banks of the anterior ectosylvian sulcus (Field AES) showed a robust anatomical projection to the SC. These data were confirmed physiologically: auditory neurons in Field AES projected to the SC and auditory SC neurons responded to stimulation of the Field AES. However, neither anatomical nor physiological techniques revealed a clear topographic relationship between the Field AES and the SC but suggested instead a diffuse and extremely divergent/convergent projection. Stimulation and cryoblockade of Field AES demonstrated the excitatory nature of this corticotectal pathway, whose influence was most evident on SC responses to stimuli of reduced intensity. Given the short latency of this ear-cortex-SC circuit and its excitatory influence on unimodal as well as on multisensory auditory neurons, it seems likely that Field AES plays a significant role in facilitating SC responses to auditory stimuli.

Neurons and behavior: the same rules of multisensory integration apply

Combinations of different sensory cues (e.g. auditory and visual) that are coincident in space enhance the responses of multisensory superior colliculus neurons, while the responses of these same neurons are depressed if the stimuli are separated in space. Using a behavioral paradigm modeled after that used in physiological studies, the present experiments demonstrate that the rules governing multisensory integration at the level of the single neuron also predict the responses to these stimuli in the intact behaving animal.

Determinants of multisensory integration in superior colliculus neurons. I. Temporal factors

One of the most impressive features of the central nervous system is its ability to process information from a variety of stimuli to produce an integrated, comprehensive representation of the external world. In the present study, the temporal disparity among combinations of different sensory stimuli was shown to be a critical factor influencing the integration of multisensory stimuli by superior colliculus neurons. Several temporal principles that govern multisensory integration were revealed: (1) maximal levels of response enhancement were generated by overlapping the peak discharge periods evoked by each modality; (2) the magnitude of this enhancement decayed monotonically to zero as the peak discharge periods became progressively more temporally disparate; (3) with further increases in temporal disparity, the same stimulus combinations that previously produced enhancement could often produce depression; and (4) these kinds of interactions could frequently be predicted from the discharge trains initiated by each stimulus alone. Since multisensory superior colliculus neurons project to premotor areas of the brain stem and spinal cord that control the orientation of the receptor organs (eyes, pinnae, head), they are believed to influence attentive and orientation behaviors. Therefore, it is likely that the temporal relationships of different environmental stimuli that control the activity of these neurons are also a powerful determinant of superior colliculus-mediated attentive and orientation behaviors.

Visual, auditory, and somatosensory convergence on cells in superior colliculus results in multisensory integration

Convergence of inputs from different sensory modalities onto individual neurons is a phenomenon that occurs widely throughout the brain at many phyletic levels and appears to represent a basic neural mechanism by which an organism integrates complex environmental stimuli. In the present study, neurons in the superior colliculus (SC) were used as a model to examine how single neurons deal with simultaneous cues from different sensory modalities (e.g., visual, auditory, somatosensory). The functional result of multisensory convergence on an individual cell was determined by comparing the responses evoked from it by a combined-modality (multimodal) stimulus with those elicited by each (unimodal) component of that stimulus presented alone. Superior colliculus cells exhibited profound changes in their activity when individual sensory stimuli were combined. These "multisensory interactions" were found to be widespread among deep laminae cells and fell into one of two functional categories: response enhancement, characterized by a significant increase in the number of discharges evoked; and response depression, characterized by a significant decrease in the discharges elicited. Multisensory response interactions most often reflected a multiplicative, rather than summative, change in activity. Their absolute magnitude varied from cell to cell and, when stimulus conditions were altered, within the same cell. However, the percentage change of enhanced interactions was generally inversely related to the vigor of the responses that could be evoked by presenting each unimodal stimulus alone and suggest that the potential for response amplification was greatest when responses evoked by individual stimuli were weakest. The majority of cells exhibiting multi-sensory characteristics were demonstrated to have descending efferent projections and thus had access to premotor and motor areas of the brain stem and spinal cord involved in SC-mediated attentive and orientation behaviors. These data show that multisensory convergence provides the descending efferent cells of the SC with a dynamic response character. The responses of these cells and the SC-mediated behaviors that they underlie need not be immutably tied to the presence of any single stimulus, but can vary in response to the particular complex of stimuli present in the environment at any given moment.

Contractile differences between muscle units in the medial rectus and lateral rectus muscles in the cat

Conjugate eye movements in the horizontal plane are accomplished by the coactivation of the medial rectus (MR) muscle of one orbit and the lateral rectus (LR) muscle of the other. While control of these excursions has been thought to be effected by identical inputs to these muscles, recent studies have demonstrated that MR motoneurons receive different inputs than LR motoneurons. This raises the question of whether the character of the muscles they control are different. The present study evaluated the contractile properties of MR and LR muscle units in the cat. Based on the mechanical aspects of their contractile properties, only two physiological types of muscle units were identified within the MR and LR muscles: twitch and non-twitch muscle units. Twitch muscle units represented over 90% of the units sampled in each muscle. Significant differences in the rate-related and the tension-related contractile properties were demonstrated between MR and LR twitch muscle units. MR muscle units exhibited significantly faster twitch contractions than did LR units. The rate of stimulation at which MR units exhibited fused tetany was significantly higher than for LR units, although units from both muscles demonstrated similar rates of rise of tension at fusion. The rate of rise of tension was closely correlated to tension production (twitch and tetanus) in each muscle. However, MR muscle units demonstrated significantly weaker maximum tetanic tensions and lower tetanus-to-twitch ratios than LR units. These data indicate that while similar physiological types of muscle fibers are present within the MR and LR, MR muscle units are adapted for faster rate-related properties, whereas LR units are adapted for greater tetanic tensions. These distinctions between MR and LR muscle units, coupled with differences between the afferent inputs to their respective motoneurons, suggest that the preservation of conjugacy during horizontal gaze shifts may require a complex interaction of peripheral and central factors.

Spatial factors determine the activity of multisensory neurons in cat superior colliculus

The responses of a neuron to stimuli from one sensory modality can be profoundly influenced by inputs from other sensory modalities. The present experiments demonstrate that the nature and the magnitude of these multisensory interactions depend on the positions of the stimuli in relation to their respective receptive fields. The spatial rules governing these interactions underscore the significance of the alignment of sensory maps in the brain.

Descending efferents from the superior colliculus relay integrated multisensory information

By means of their efferent projections to motor and premotor structures, the cells in the deep superior colliculus are intimately involved in behaviors that control the orientation of the eyes, pinnae, and head. These same efferent cells receive multiple sensory inputs, thereby apparently enabling an animal to orient its receptor organs in response to a wide variety of cues. This sensory convergence also provides a system in which motor responses need not be immutably linked to individual stimuli but can vary in reaction to the multitude of stimuli present in the environment at any given moment.

Interactions among converging sensory inputs in the superior colliculus

The responses of superior colliculus cells to a given sensory stimulus were influenced by the presence or absence of other sensory cues. By pooling sensory inputs, many superior colliculus cells seem to amplify the effects of subtle environmental cues in certain conditions, whereas in others, responses to normally effective stimuli can be blocked. The observations illustrate the dynamic, interactive nature of the multisensory inputs which characterize the deeper laminae of the superior colliculus.

Retractor bulbi muscle responses to oculomotor nerve and nucleus stimulation in the cat

This study demonstrates the presence of retractor bulbi motoneurons within the oculomotor nucleus which activate muscle units within all 4 slips of the cat retractor bulbi muscle. These muscle units are mechanically different and physiologically separate from retractor bulbi muscle units innervated by the abducens nerve. The refractor bulbi muscle, then, is innervated by two separate pools of motoneurons whose axons are carried in two different cranial nerves. These observations of mechanical properties of retractor bulbi muscle suggest that the oculomotor retractor bulbi motor units may be activated during patterned eye movements.

The cardiac malformations. Double inlet left ventricle and corrected transposition explained as deviations in the normal development of the interventricular septum

To examine the hypothesis that malpositions of cardiac ventricles could be explained by altered development of the interventricular septum, we studied hearts from the Johns Hopkins Hospital autopsy files with double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet ventricle both atrioventricular valves connect normally developed and positioned right and left atria to a posterior morphologic left ventricle. In hearts with corrected transposition the atria are normally positioned and the morphologic right ventricle is on the left and is continuous with the anteriorly positioned aorta. The morphologic left ventricle is on the right, connected to the posteriorly positioned pulmonary trunk. Normal ventricular septation may be understood as arising from the mechanics of a spiral fold in the primary heart tube produced by the left interventricular sulcus. The ventral limb of the spiral induces the muscular interventricular septum while the dorsal limb becomes a component of the crista supraventricularis. We propose that double inlet left ventricle and corrected transposition are the result of minor deviations in the position of the interventricular sulcus on the primary heart tube. Double inlet left ventricle may develop from the formation of a closed unspiraled ring around the interventricular canal. Corrected transposition may result from a left interventricular sulcus whose ventral limb gives rise to a left sided crista supraventricularis, which determines in part the right ventricular morphology of the left sided ventricle. The dorsal limb spirals toward th atrioventricular canal, gives rise to a malpositioned interventricular septum, and displaces the embryonic trabeculated right ventricle to the left. The concept presented accounts for the morphologic findings characteristic of double inlet left ventricle and corrected transposition.

Role of the left interventricular sulcus in formation of interventricular septum and crista supraventricularis in normal human cardiogenesis

Serial sections of normal human embryos were studied and three-dimensional images reconstructed to determine the early development of the interventricular septum. The position of the interventricular septum is determined in stage 9 of normal development by the formation of the left interventricular sulcus. As a result of unknown properties of the cells of the myocardial layer, the left interventricular sulcus persists while the right disappears, producing the initial lateral asymmetry of the primary heart tube. By stage 14, the left interventricular sulcus forms a spiral which is continuous with the developing interventricular septum. The dorsal limb of the spiral passes to the right between the atrioventricular canal and the origin of the outflow tract, and is lost in the wall of the trabeculated right ventricle. It appears that this dorsal limb of the spiral is the precursor of part of the cirsta supraventricularis. The midportion of the sulcus, the bulboventricular groove, becomes the so-called fibrous continuity between the aortic and mitral valves. The ventral limb of the spiral passes caudally in the anterior interventricular groove and then dorsally and cranially toward the dorsal cushion of the atrioventricular canal. The ventral limb of the spiral is continuous with the crest of the muscular interventricular septum, which develops by apposition of tissue from the expanding right and left ventricles. From stage 14 to stage 19, the muscular interventricular septum, the atrioventricular endocardial cushions, and the ventricular end of the spiral ridges of the outflow tract appose and fuse. Subsequent formation of the membranous interventricular septum completes the physical separation of the right and left ventricles.