A motion-sensitive area in ferret extrastriate visual cortex: an analysis in pigmented and albino animals

Neocortical pyramidal neurons with axons emerging from dendrites are frequent in non-primates, but rare in monkey and human

The canonical view of neuronal function is that inputs are received by dendrites and somata, become integrated in the somatodendritic compartment and upon reaching a sufficient threshold, generate axonal output with axons emerging from the cell body. The latter is not necessarily the case. Instead, axons may originate from dendrites. The terms ‘axon carrying dendrite’ (AcD) and ‘AcD neurons’ have been coined to describe this feature. In rodent hippocampus, AcD cells are shown to be functionally ‘privileged’, since inputs here can circumvent somatic integration and lead to immediate action potential initiation in the axon. Here, we report on the diversity of axon origins in neocortical pyramidal cells of rodent, ungulate, carnivore, and primate. Detection methods were Thy-1-EGFP labeling in mouse, retrograde biocytin tracing in rat, cat, ferret, and macaque, SMI-32/βIV-spectrin immunofluorescence in pig, cat, and macaque, and Golgi staining in macaque and human. We found that in non-primate mammals, 10–21% of pyramidal cells of layers II–VI had an AcD. In marked contrast, in macaque and human, this proportion was lower and was particularly low for supragranular neurons. A comparison of six cortical areas (being sensory, association, and limbic in nature) in three macaques yielded percentages of AcD cells which varied by a factor of 2 between the areas and between the individuals. Unexpectedly, pyramidal cells in the white matter of postnatal cat and aged human cortex exhibit AcDs to much higher percentages. In addition, interneurons assessed in developing cat and adult human cortex had AcDs at type-specific proportions and for some types at much higher percentages than pyramidal cells. Our findings expand the current knowledge regarding the distribution and proportion of AcD cells in neocortex of non-primate taxa, which strikingly differ from primates where these cells are mainly found in deeper layers and white matter.

Morphological evidence for multiple distinct channels of corticogeniculate feedback originating in mid-level extrastriate visual areas of the ferret

Complementary reciprocal feedforward and feedback circuits connecting the visual thalamus with the visual cortex are essential for visual perception. These circuits predominantly connect primary and secondary visual cortex with the dorsal lateral geniculate nucleus (LGN). Although there are direct geniculocortical inputs to extrastriate visual cortex, whether reciprocal corticogeniculate neurons exist in extrastriate cortex is not known. Here we utilized virus-mediated retrograde tracing to reveal the presence of corticogeniculate neurons in three mid-level extrastriate visual cortical areas in ferrets: PMLS, PLLS, and 21a. We observed corticogeniculate neurons in all three extrastriate areas, although the density of virus-labeled corticogeniculate neurons in extrastriate cortex was an order of magnitude less than that in areas 17 and 18. A cluster analysis of morphological metrics quantified following reconstructions of the full dendritic arborizations of virus-labeled corticogeniculate neurons revealed six distinct cell types. Similar corticogeniculate cell types to those observed in areas 17 and 18 were also observed in PMLS, PLLS, and 21a. However, these unique cell types were not equally distributed across the three extrastriate areas. The majority of corticogeniculate neurons per cluster originated in a single area, suggesting unique parallel organizations for corticogeniculate feedback from each extrastriate area to the LGN. Together, our findings demonstrate direct feedback connections from mid-level extrastriate visual cortex to the LGN, supporting complementary reciprocal circuits at multiple processing stages along the visual hierarchy. Importantly, direct reciprocal connections between the LGN and extrastriate cortex, that bypass V1, could provide a substrate for residual vision following V1 damage.

The Ferret as a Model System for Neocortex Development and Evolution

The neocortex is the largest part of the cerebral cortex and a key structure involved in human behavior and cognition. Comparison of neocortex development across mammals reveals that the proliferative capacity of neural stem and progenitor cells and the length of the neurogenic period are essential for regulating neocortex size and complexity, which in turn are thought to be instrumental for the increased cognitive abilities in humans. The domesticated ferret, Mustela putorius furo, is an important animal model in neurodevelopment for its complex postnatal cortical folding, its long period of forebrain development and its accessibility to genetic manipulation in vivo. Here, we discuss the molecular, cellular, and histological features that make this small gyrencephalic carnivore a suitable animal model to study the physiological and pathological mechanisms for the development of an expanded neocortex. We particularly focus on the mechanisms of neural stem cell proliferation, neuronal differentiation, cortical folding, visual system development, and neurodevelopmental pathologies. We further discuss the technological advances that have enabled the genetic manipulation of the ferret in vivo. Finally, we compare the features of neocortex development in the ferret with those of other model organisms.

Stimulus-specific regulation of visual oddball differentiation in posterior parietal cortex

The frequency at which a stimulus is presented determines how it is interpreted. For example, a repeated image may be of less interest than an image that violates the prior sequence. This process involves integration of sensory information and internal representations of stimulus history, functions carried out in higher-order sensory areas such as the posterior parietal cortex (PPC). Thus far, there are few detailed reports investigating the single-neuron mechanisms for processing of stimulus presentation frequency in PPC. To address this gap in knowledge, we recorded PPC activity using 2-photon calcium imaging and electrophysiology during a visual oddball paradigm. Calcium imaging results reveal differentiation at the level of single neurons for frequent versus rare conditions which varied depending on whether the stimulus was preferred or non-preferred by the recorded neural population. Such differentiation of oddball conditions was mediated primarily by stimulus-independent adaptation in the frequent condition.

Visual Motion and Form Integration in the Behaving Ferret

Ferrets have become a standard animal model for the development of early visual stages. Less is known about higher-level vision in ferrets, both during development and in adulthood. Here, as a step towards establishing higher-level vision research in ferrets, we used behavioral experiments to test the motion and form integration capacity of adult ferrets. Motion integration was assessed by training ferrets to discriminate random dot kinematograms (RDK) based on their direction. Task difficulty was varied systematically by changing RDK coherence levels, which allowed the measurement of motion integration thresholds. Form integration was measured analogously by training ferrets to discriminate linear Glass patterns of varying coherence levels based on their orientation. In all experiments, ferrets proved to be good psychophysical subjects that performed tasks reliably. Crucially, the behavioral data showed clear evidence of perceptual motion and form integration. In the monkey, motion and form integration are usually associated with processes occurring in higher-level visual areas. In a second set of experiments, we therefore tested whether PSS, a higher-level motion area in the ferret, could similarly support motion integration behavior in this species. To this end, we measured responses of PSS neurons to RDK of different coherence levels. Indeed, neurometric functions for PSS were in good agreement with the behaviorally derived psychometric functions. In conclusion, our experiments demonstrate that ferrets are well suited for higher-level vision research.

Ferrets as a Model for Higher-Level Visual Motion Processing

Ferrets are a major developmental animal model due to their early parturition. Here we show for the first time that ferrets could be used to study development of higher-level visual processes previously identified in primates. In primates, complex motion processing involves primary visual cortex (V1), which generates local motion signals, and higher-level visual area MT, which integrates these signals over more global spatial regions. Our data show similar transformations in motion signals between ferret V1 and higher-level visual area PSS, located in the posterior bank of the suprasylvian sulcus. We found that PSS neurons, like MT neurons, were tuned for stimulus motion and showed strong suppression between opposing direction inputs. Most strikingly, PSS, like MT, exhibited robust global motion signals when tested with coherent plaids-the classic test for motion integration across multiple moving elements. These PSS responses were described well by computational models developed for MT. Our findings establish the ferret as a strong animal model for development of higher-level visual processing.

Cortico-Cortical Connectivity Within Ferret Auditory Cortex

Despite numerous studies of auditory cortical processing in the ferret (Mustela putorius), very little is known about the connections between the different regions of the auditory cortex that have been characterized cytoarchitectonically and physiologically. We examined the distribution of retrograde and anterograde labeling after injecting tracers into one or more regions of ferret auditory cortex. Injections of different tracers at frequency‐matched locations in the core areas, the primary auditory cortex (A1) and anterior auditory field (AAF), of the same animal revealed the presence of reciprocal connections with overlapping projections to and from discrete regions within the posterior pseudosylvian and suprasylvian fields (PPF and PSF), suggesting that these connections are frequency specific. In contrast, projections from the primary areas to the anterior dorsal field (ADF) on the anterior ectosylvian gyrus were scattered and non‐overlapping, consistent with the non‐tonotopic organization of this field. The relative strength of the projections originating in each of the primary fields differed, with A1 predominantly targeting the posterior bank fields PPF and PSF, which in turn project to the ventral posterior field, whereas AAF projects more heavily to the ADF, which then projects to the anteroventral field and the pseudosylvian sulcal cortex. These findings suggest that parallel anterior and posterior processing networks may exist, although the connections between different areas often overlap and interactions were present at all levels. J. Comp. Neurol. 523:2187–2210, 2015. © 2015 Wiley Periodicals, Inc.

Retinal ganglion cells projecting to the accessory optic system in optokinetic blind albinotic rats are direction-selective

The optokinetic deficits in albinotic rats and ferrets are caused by the loss of direction selectivity in the accessory optic system (AOS). However, the underlying mechanisms for this loss are still not clear. Here we tested the hypothesis that, in albino rats, the retinal input to the AOS lacks direction selectivity and, as a consequence, neurons in the AOS are direction non-selective. We investigated ON-center direction-selective retinal ganglion cells, the major input to the AOS, in pigmented Long Evans and albino Wistar rats using extracellular in vitro patch-clamp techniques. To visualise putative AOS-projecting direction-selective ganglion cells, we retrogradely labeled them by injection of the infrared-sensitive dye indocyanine green into the medial terminal nucleus of the AOS. The present study is the first to present physiological evidence for retinal ON-center direction-selective ganglion cells in rat. Our results show that, in albinotic and pigmented rats, ON-center retinal ganglion cells projecting to the AOS are similarly direction-selective, suggesting that the optokinetic deficit must be caused by the abolition of direction selectivity in the AOS itself.

Development of cerebral sulci and gyri in ferrets (Mustela putorius)

The present study aimed to clarify sulcation and gyration patterns in the developing cerebrum of ferrets. While the brain weight and fronto-occipital length of the cerebral hemisphere reached a plateau by postnatal day (PD) 42, the cerebral width reached a plateau at the rostral region by PD 21, and subsequently at the caudal region by PD 42. The ferret cerebrum already showed a convoluted surface with indentations of coronal and rostral suprasylvian sulci on PD 4. The presylvian and cruciate sulci emerged by PD 10, resulting in convolutions of gyri in the rostral half of the cerebrum. The caudal half of the cerebrum was infolded by the emergence of the pseudosylvian sulcus and the rhinal fissure by PD 10, and the caudal suprasylvian and lateral sulci by PD 21. The emergence of those sulci allowed a gyration in the caudal half of the cerebrum. Sexual differences in sulcation were detected by a more distinct convolution of the visual cortex in males than in females on PD 90. Those results, therefore, suggest that the ferret cerebrum experiences cortical maturation with sulcation and gyration in a rostrocaudal gradient manner. The present paper provides neuroanatomic references for normal development of cerebral sulci and gyri in both sexes of ferrets.

Functional biases in visual cortex neurons with identified projections to higher cortical targets

BACKGROUND

Visual perception involves information flow from lower- to higher-order cortical areas, which are known to process different kinds of information. How does this functional specialization arise? As a step toward addressing this question, we combined fluorescent retrograde tracing with in vivo two-photon calcium imaging to simultaneously compare the tuning properties of neighboring neurons in areas 17 and 18 of ferret visual cortex that have different higher cortical projection targets.

RESULTS

Neurons projecting to the posterior suprasylvian sulcus (PSS) were more direction selective and preferred shorter stimuli, higher spatial frequencies, and higher temporal frequencies than neurons projecting to area 21, anticipating key differences between the functional properties of the target areas themselves. These differences could not be explained by a correspondence between anatomical and functional clustering within early visual cortex, and the largest differences were in properties generated within early visual cortex (direction selectivity and length preference) rather than in properties present in its retinogeniculate inputs.

CONCLUSIONS

These projection cell groups, and hence the higher-order visual areas to which they project, likely obtain their functional properties not from biased retinogeniculate inputs but from highly specific circuitry within the cortex.

Immunohistochemical parcellation of the ferret (Mustela putorius) visual cortex reveals substantial homology with the cat (Felis catus)

Electrophysiological mapping of the adult ferret visual cortex has until now determined the existence of 12 retinotopically distinct areas; however, in the cat, another member of the Carnivora, 20 distinct visual areas have been identified by using retinotopic mapping and immunolabeling. In the present study, the immunohistochemical approach to demarcate the areal boundaries of the adult ferret visual cortex was applied in order to overcome the difficulties in accessing the sulcal surfaces of a small, gyrencephalic brain. Nonphosphorylated neurofilament (NNF) expression profiles were compared with another classical immunostain of cortical nuclei, Cat-301 chondroitin sulfate proteoglycan (CSPG). Together, these two markers reliably demarcated the borders of the 12 previously defined areas and revealed further arealization beyond those borders to a total of 19 areas: 21a and 21b; the anterolateral, posterolateral, dorsal, and ventral lateral suprasylvian areas (ALLS, PLLS, DLS, and VLS, respectively); and the splenial and cingulate visual areas (SVA and CVA). NNF expression profile and location of the newly defined areas correlate with previously defined areas in the cat. Moreover, NNF and Cat-301 together revealed discrete expression domains in the posteroparietal (PP) cortex, demarcating four subdivisions in the caudal lateral and medial domains (PPcL and PPcM) and rostral lateral and medial domains (PPrL and PPrM), where only two retinotopic maps have been previously identified (PPc and PPr). Taken together, these studies suggest that NNF and Cat-301 can illustrate the homology between cortical areas in different species and draw out the principles that have driven evolution of the visual cortex.

Six principles of visual cortical dynamics

A fundamental goal in vision science is to determine how many neurons in how many areas are required to compute a coherent interpretation of the visual scene. Here I propose six principles of cortical dynamics of visual processing in the first 150 ms following the appearance of a visual stimulus. Fast synaptic communication between neurons depends on the driving neurons and the biophysical history and driving forces of the target neurons. Under these constraints, the retina communicates changes in the field of view driving large populations of neurons in visual areas into a dynamic sequence of feed-forward communication and integration of the inward current of the change signal into the dendrites of higher order area neurons (30-70 ms). Simultaneously an even larger number of neurons within each area receiving feed-forward input are pre-excited to sub-threshold levels. The higher order area neurons communicate the results of their computations as feedback adding inward current to the excited and pre-excited neurons in lower areas. This feedback reconciles computational differences between higher and lower areas (75-120 ms). This brings the lower area neurons into a new dynamic regime characterized by reduced driving forces and sparse firing reflecting the visual areas interpretation of the current scene (140 ms). The population membrane potentials and net-inward/outward currents and firing are well behaved at the mesoscopic scale, such that the decoding in retinotopic cortical space shows the visual areas' interpretation of the current scene. These dynamics have plausible biophysical explanations. The principles are theoretical, predictive, supported by recent experiments and easily lend themselves to experimental tests or computational modeling.

The non-lemniscal auditory cortex in ferrets: convergence of corticotectal inputs in the superior colliculus

Descending cortical inputs to the superior colliculus (SC) contribute to the unisensory response properties of the neurons found there and are critical for multisensory integration. However, little is known about the relative contribution of different auditory cortical areas to this projection or the distribution of their terminals in the SC. We characterized this projection in the ferret by injecting tracers in the SC and auditory cortex. Large pyramidal neurons were labeled in layer V of different parts of the ectosylvian gyrus after tracer injections in the SC. Those cells were most numerous in the anterior ectosylvian gyrus (AEG), and particularly in the anterior ventral field, which receives both auditory and visual inputs. Labeling was also found in the posterior ectosylvian gyrus (PEG), predominantly in the tonotopically organized posterior suprasylvian field. Profuse anterograde labeling was present in the SC following tracer injections at the site of acoustically responsive neurons in the AEG or PEG, with terminal fields being both more prominent and clustered for inputs originating from the AEG. Terminals from both cortical areas were located throughout the intermediate and deep layers, but were most concentrated in the posterior half of the SC, where peripheral stimulus locations are represented. No inputs were identified from primary auditory cortical areas, although some labeling was found in the surrounding sulci. Our findings suggest that higher level auditory cortical areas, including those involved in multisensory processing, may modulate SC function via their projections into its deeper layers.

The role of multi-area interactions for the computation of apparent motion

Apparent motion (AM) is a robust visual illusion, in which fast displays of static objects in successively different positions elicit the perception of object motion. Neurons in higher order areas 21 and 19 compute object motion under such conditions and send feedback to early visual areas 18 and 17, which is instrumental in eliciting computation of motion in those very areas. To explore the computational dynamics of AM, we made a neural field model consisting of two one-dimensional rings of simple neurons expressing firing rates, one for areas 17/18 and one for areas 19/21. The model neurons, without any orientation or direction selectivity, computed apparent motion for the range of space-timings of stimuli associated with short- and long-range AM in humans. The computation of long-range AM in 17/18 required two model areas and the presence of feedback and conduction/computation delays between those areas. As in the in vivo experiments of long-range AM, the stationary stimuli were initially mapped as stationary in model area 17/18, but after the feedback also these lower areas computed AM. The dynamics of the two-area network produces short-range and long-range apparent motion for a large range of feedback strengths and a small range of lateral excitation near the bifurcation to an amplitude instability. The computation of AM in higher order areas was due to the neurons in these areas having large receptive fields as a consequence of divergent feed-forward connectivity. This implies that these areas compute long-range AM when early areas 17 and 18 do not, and therefore higher order areas must enslave lower order areas to compute the same, if the whole network is to arrive at a coherent perceptual solution.

Visual influences on ferret auditory cortex

Multisensory neurons are now known to be widespread in low-level regions of the cortex usually thought of as being responsible for modality-specific processing. The auditory cortex provides a particularly striking example of this, exhibiting responses to both visual and somatosensory stimulation. Single-neuron recording studies in ferrets have shown that each of auditory fields that have been characterized using physiological and anatomical criteria also receives visual inputs, with the incidence of visually-sensitive neurons ranging from 15% to 20% in the primary areas to around 50% or more in higher-level areas. Although some neurons exhibit spiking responses to visual stimulation, these inputs often have subthreshold influences that modulate the responses of the cortical neurons to sound. Insights into the possible role played by the visual inputs can be obtained by examining their sources of origin and the way in which they alter the processing capabilities of neurons in the auditory cortex. These studies suggest that one of the functions of the visual input to auditory cortex is to sharpen the relatively imprecise spatial coding typically found there. Because the extent to which this happens varies between cortical fields, the investigation of multisensory interactions can also help in understanding their relative contributions to auditory perception.

Retinal projections to the accessory optic system in pigmented and albino ferrets (Mustela putorius furo)

We investigated if a reduced specificity of the retinal projection to the accessory optic system might be responsible for the loss of direction selectivity in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) and, in consequence of this, the optokinetic deficits in albino ferrets. Under electrophysiological control we performed dual tracer injections into the NOT-DTN and the medial terminal nucleus (MTN). Retrogradely labelled ganglion cells were found in the visual streak, the dorsal, and the ventral retina both after injections into the NOTDTN and the MTN indicating that both nuclei receive input from the same retinal regions. The distribution and spacing of labelled ganglion cells did not differ between pigmented and albino ferrets. However, retinal ganglion cells projecting simultaneously to both the NOT-DTN and the MTN occurred only in albino ferrets. These results suggest that a reduced specificity of the projection pattern of direction specific ganglion cells may contribute to the loss of direction selectivity in the NOT-DTN in albino ferrets.

Visual-auditory spatial processing in auditory cortical neurons

Neurons responsive to visual stimulation have now been described in the auditory cortex of various species, but their functions are largely unknown. Here we investigate the auditory and visual spatial sensitivity of neurons recorded in 5 different primary and non-primary auditory cortical areas of the ferret. We quantified the spatial tuning of neurons by measuring the responses to stimuli presented across a range of azimuthal positions and calculating the mutual information (MI) between the neural responses and the location of the stimuli that elicited them. MI estimates of spatial tuning were calculated for unisensory visual, unisensory auditory and for spatially and temporally coincident auditory-visual stimulation. The majority of visually responsive units conveyed significant information about light-source location, whereas, over a corresponding region of space, acoustically responsive units generally transmitted less information about sound-source location. Spatial sensitivity for visual, auditory and bisensory stimulation was highest in the anterior dorsal field, the auditory area previously shown to be innervated by a region of extrastriate visual cortex thought to be concerned primarily with spatial processing, whereas the posterior pseudosylvian field and posterior suprasylvian field, whose principal visual input arises from cortical areas that appear to be part of the 'what' processing stream, conveyed less information about stimulus location. In some neurons, pairing visual and auditory stimuli led to an increase in the spatial information available relative to the most effective unisensory stimulus, whereas, in a smaller subpopulation, combined stimulation decreased the spatial MI. These data suggest that visual inputs to auditory cortex can enhance spatial processing in the presence of multisensory cues and could therefore potentially underlie visual influences on auditory localization.

Elevated intracellular chloride level in albino visual cortex neurons is mediated by Na-K-Cl co-transporter

Background

During development the switch from a depolarizing to a hyperpolarizing action of GABA is a consequence of a decrease of the Na⁺-K⁺-2Cl^- co-transporter (NKCC1, Cl^--uptake) and increase of the K⁺-Cl^- co-transporter (KCC2, Cl^--extrusion) expression. However albino visual cortex neurons don't show a corresponding decrease in intracellular chloride concentration during development of the visual system as compared to pigmented animals.

Results

Our study revealed that more cells express NKCC1 in albinos compared to pigmented rat visual cortex neurons whereas KCC2 is expressed in all cells in both strains. We determined a positive relationship between the presence of NKCC1 and an inhibitory deficit in single neurons of the albino visual cortex. After pharmacological blockade of NKCC1 function with its specific inhibitor, bumetanide, the reversal potential of electrically evoked GABAA receptor-mediated postsynaptic currents and, as a consequence, [Cl-]_i in albino visual cortex neurons shifted to the pigmented rat brain value. In conclusion, our pharmacological experiments and subsequent single cell real time PCR analysis of the co-transporter mRNA demonstrated that the inhibitory deficit present in the albino visual cortical network is almost exclusively mediated by NKCC1.

Conclusion

Our findings suggest that blocking of NKCC1 in albino visual cortex neurons could improve processing in visual cortex and therefore might be beneficial for vision in albinos.

Cortical dynamics subserving visual apparent motion

Motion can be perceived when static images are successively presented with a spatial shift. This type of motion is an illusion and is termed apparent motion (AM). Here we show, with a voltage sensitive dye applied to the visual cortex of the ferret, that presentation of a sequence of stationary, short duration, stimuli which are perceived to produce AM are, initially, mapped in areas 17 and 18 as separate stationary representations. But time locked to the offset of the 1st stimulus, a sequence of signals are elicited. First, an activation traverses cortical areas 19 and 21 in the direction of AM. Simultaneously, a motion dependent feedback signal from these areas activates neurons between areas 19/21 and areas 17/18. Finally, an activation is recorded, traveling always from the representation of the 1st to the representation of the next or succeeding stimuli. This activation elicits spikes from neurons situated between these stimulus representations in areas 17/18. This sequence forms a physiological mechanism of motion computation which could bind populations of neurons in the visual areas to interpret motion out of stationary stimuli.

Deficits of visual motion perception and optokinetic nystagmus after posterior suprasylvian lesions in the ferret (Mustela putorius furo)

We recently described an area in the ferret posterior suprasylvian (PSS) cortex characterized by a high proportion of direction selective neurons. To answer the question whether area PSS subserves functions similar to cat posteromediolateral suprasylvian area (PMLS) and monkey medial temporal area (MT) we investigated the contribution of area PSS to visual motion perception and optokinetic nystagmus. Ferrets were tested on global motion detection before and after bilateral lesions involving area PSS and control lesions of other extrastriate visual areas. Following PSS lesions motion coherence thresholds were significantly increased both in pigmented and albino ferrets, whereas control lesions sparing PSS did not affect visual motion perception. Optokinetic nystagmus was strongly reduced to absent after PSS lesions. These results indicate that area PSS is crucial for global motion processing in the ferret and in that sense may be functionally equivalent to PMLS in the cat and area MT in the monkey.

Physiological and anatomical evidence for multisensory interactions in auditory cortex

Recent studies, conducted almost exclusively in primates, have shown that several cortical areas usually associated with modality-specific sensory processing are subject to influences from other senses. Here we demonstrate using single-unit recordings and estimates of mutual information that visual stimuli can influence the activity of units in the auditory cortex of anesthetized ferrets. In many cases, these units were also acoustically responsive and frequently transmitted more information in their spike discharge patterns in response to paired visual-auditory stimulation than when either modality was presented by itself. For each stimulus, this information was conveyed by a combination of spike count and spike timing. Even in primary auditory areas (primary auditory cortex [A1] and anterior auditory field [AAF]), approximately 15% of recorded units were found to have nonauditory input. This proportion increased in the higher level fields that lie ventral to A1/AAF and was highest in the anterior ventral field, where nearly 50% of the units were found to be responsive to visual stimuli only and a further quarter to both visual and auditory stimuli. Within each field, the pure-tone response properties of neurons sensitive to visual stimuli did not differ in any systematic way from those of visually unresponsive neurons. Neural tracer injections revealed direct inputs from visual cortex into auditory cortex, indicating a potential source of origin for the visual responses. Primary visual cortex projects sparsely to A1, whereas higher visual areas innervate auditory areas in a field-specific manner. These data indicate that multisensory convergence and integration are features common to all auditory cortical areas but are especially prevalent in higher areas.

Motion perception deficits in albino ferrets (Mustela putorius furo)

Albino ferrets contrary to their pigmented conspecifics show no optokinetic nystagmus. Therefore, in this study motion perception was compared between pigmented and albino ferrets (Mustela putorius furo) trained to discriminate between coherently moving random dot patterns and dynamic noise stimuli in a two-alternative forced choice task. Fully coherently versus incoherently moving patterns could be distinguished by ferrets of both phenotypes. Motion coherence thresholds, however, were significantly higher in albinos. These results indicate that albino ferrets are not motion blind as could be expected from their total lack of optokinetic reactions. However, they are severely impaired in global motion perception.

Retinotopic organization of ferret suprasylvian cortex

The retinotopic organization of striate and several extrastriate areas of ferret cortex has been established. Here we describe the representation of the visual field on the Suprasylvian visual area (Ssy). This cortical region runs mediolaterally along the posterior bank of the suprasylvian sulcus, and is distinct from adjoining areas in anatomical architecture. The Ssy lies immediately rostral to visual area 21, medial to lateral temporal areas, and lateral to posterior parietal areas. In electrophysiological experiments we made extracellular recordings in adult ferrets. We find that single and multiunit receptive fields range in size from 2 deg x 4 deg to 21 deg x 52 deg. The total visual field representation in Ssy spans over 70 deg in azimuth in the contralateral hemifield (with a small incursion into the ipsilateral hemifield), and from +36 deg to -30 deg in elevation. There are often two representations of the horizontal meridian. Furthermore, the location of the transition from upper to lower fields varies among animals. General features of topography are confirmed in anatomical experiments in which we made tracer injections into different locations in Ssy, and determined the location of retrograde label in area 17. Both isoelevation and isoazimuth lines can span substantial rostrocaudal and mediolateral distances in cortex, sometimes forming closed contours. This topography results in cortical magnifications averaging 0.07 mm/deg in elevation and 0.06 mm/deg in azimuth; however, some contours can run in such a way that it is possible to move a large distance on cortex without moving in the visual field. Because of these irregularities, Ssy contains a coarse representation of the contralateral visual field.