Receptive field organization of 'sustained' and 'transient' retinal ganglion cells which subserve different function roles

Visual hallucinations originating in the retinofugal pathway under clinical and psychedelic conditions

Psychedelics like LSD (Lysergic acid diethylamide) and psilocybin are known to modulate perceptual modalities due to the activation of mostly serotonin receptors in specific cortical (e.g., visual cortex) and subcortical (e.g., thalamus) regions of the brain. In the visual domain, these psychedelic modulations often result in peculiar disturbances of viewed objects and light and sometimes even in hallucinations of non-existent environments, objects, and creatures. Although the underlying processes are poorly understood, research conducted over the past twenty years on the subjective experience of psychedelics details theories that attempt to explain these perceptual alterations due to a disruption of communication between cortical and subcortical regions. However, rare medical conditions in the visual system like Charles Bonnet syndrome that cause perceptual distortions may shed new light on the additional importance of the retinofugal pathway in psychedelic subjective experiences. Interneurons in the retina called amacrine cells could be the first site of visual psychedelic modulation and aid in disrupting the hierarchical structure of how humans perceive visual information. This paper presents an understanding of how the retinofugal pathway communicates and modulates visual information in psychedelic and clinical conditions. Therefore, we elucidate a new theory of psychedelic modulation in the retinofugal pathway.

Gap junctions fine-tune ganglion cell signals to equalize response kinetics within a given electrically coupled array

Retinal ganglion cells (RGCs) summate inputs and forward a spike train code to the brain in the form of either maintained spiking (sustained) or a quickly decaying brief spike burst (transient). We report diverse response transience values across the RGC population and, contrary to the conventional transient/sustained scheme, responses with intermediary characteristics are the most abundant. Pharmacological tests showed that besides GABAergic inhibition, gap junction (GJ)-mediated excitation also plays a pivotal role in shaping response transience and thus visual coding. More precisely GJs connecting RGCs to nearby amacrine and RGCs play a defining role in the process. These GJs equalize kinetic features, including the response transience of transient OFF alpha (tOFFα) RGCs across a coupled array. We propose that GJs in other coupled neuron ensembles in the brain are also critical in the harmonization of response kinetics to enhance the population code and suit a corresponding task.

Starburst amacrine cells amplify optogenetic visual restoration through gap junctions

Ectopic induction of optogenetic actuators, such as channelrhodopsin, is a promising approach to restoring vision in the degenerating retina. However, the cell type-specific response of ectopic photoreception has not been well understood. There are limits to obtaining efficient gene expression in a specifically targeted cell population by a transgenic approach. In the present study, we established a murine model with high efficiency of gene induction to retinal ganglion cells (RGCs) and amacrine cells using an improved tetracycline transactivator-operator bipartite system (KENGE-tet system). To investigate the cell type-specific visual restorative effect, we expressed the channelrhodopsin gene into RGCs and amacrine cells using the KENGE-tet system. As a result, enhancement in the visual restorative effect was observed to RGCs and starburst amacrine cells. In conclusion, a photoresponse from amacrine cells may enhance the maintained response of RGCs and further increase or improve the visual restorative effect.

Optogenetic Suppression of Lateral Septum Somatostatin Neurons Enhances Hippocampus Cholinergic Theta Oscillations and Local Synchrony

The septal complex regulates both motivated and innate behaviors, chiefly by the action of its diverse population of long-range projection neurons. A small population of somatostatin-expressing GABAergic cells in the lateral septum projects deep into subcortical regions, yet on its way it also targets neighboring medial septum neurons that profusely innervate cortical targets by ascending synaptic pathways. Here, we used optogenetic stimulation and extracellular recordings in acutely anesthetized transgenic mice to show that lateral septum somatostatin neurons can disinhibit the cholinergic septo-hippocampal pathway, thus enhancing the amplitude and synchrony of theta oscillations while depressing sharp-wave ripple episodes in the dorsal hippocampus. These results suggest that septal somatostatin cells can recruit ascending cholinergic pathways to promote hippocampal theta oscillations.

Dynamics of absolute and relative disparity processing in human visual cortex

Cortical processing of binocular disparity is believed to begin in V1 where cells are sensitive to absolute disparity, followed by the extraction of relative disparity in higher visual areas. While much is known about the cortical distribution and spatial tuning of disparity-selective neurons, the relationship between their spatial and temporal properties is less well understood. Here, we use steady-state Visual Evoked Potentials and dynamic random dot stereograms to characterize the temporal dynamics of spatial mechanisms in human visual cortex that are primarily sensitive to either absolute or relative disparity. Stereograms alternated between disparate and non-disparate states at 2 Hz. By varying the disparity-defined spatial frequency content of the stereograms from a planar surface to corrugated ones, we biased responses towards absolute vs. relative disparities. Reliable Components Analysis was used to derive two dominant sources from the 128 channel EEG records. The first component (RC1) was maximal over the occipital pole. In RC1, first harmonic responses were sustained, tuned for corrugation frequency, and sensitive to the presence of disparity references, consistent with prior psychophysical sensitivity measurements. By contrast, the second harmonic, associated with transient processing, was not spatially tuned and was indifferent to references, consistent with it being generated by an absolute disparity mechanism. Thus, our results reveal a duplex coding strategy in the disparity domain, where relative disparities are computed via sustained mechanisms and absolute disparities are computed via transient mechanisms.

Transience of the Retinal Output Is Determined by a Great Variety of Circuit Elements

Retinal ganglion cells (RGCs) encrypt stimulus features of the visual scene in action potentials and convey them toward higher visual centers in the brain. Although there are many visual features to encode, our recent understanding is that the ~46 different functional subtypes of RGCs in the retina share this task. In this scheme, each RGC subtype establishes a separate, parallel signaling route for a specific visual feature (e.g., contrast, the direction of motion, luminosity), through which information is conveyed. The efficiency of encoding depends on several factors, including signal strength, adaptational levels, and the actual efficacy of the underlying retinal microcircuits. Upon collecting inputs across their respective receptive field, RGCs perform further analysis (e.g., summation, subtraction, weighting) before they generate the final output spike train, which itself is characterized by multiple different features, such as the number of spikes, the inter-spike intervals, response delay, and the rundown time (transience) of the response. These specific kinetic features are essential for target postsynaptic neurons in the brain in order to effectively decode and interpret signals, thereby forming visual perception. We review recent knowledge regarding circuit elements of the mammalian retina that participate in shaping RGC response transience for optimal visual signaling.

Parallel processing in human visual cortex revealed through the influence of their neural responses on the visual evoked potential

The neural response in the human visual system is composed of magno-, parvo- and koniocellular input from the retina. Signal differences from functional imaging between health and individuals with a cognitive weakness are attributed to a dysfunction of a specific retinal input. Yet, anatomical interconnections within the human visual system obscure individual contribution to the neural response in V1. Deflections in the visual evoked potential (VEP) arise from an interaction between electric dipoles, their strength determined by the size of the neural population active during temporal - and spatial luminance contrast processing. To investigate interaction between these neural responses, we recorded the VEP over visual cortex of 14 healthy adults viewing four series of windmill patterns. Within a series, the relative area white in a pattern varied systematically. Between series, the number of sectors across which this area was distributed doubled. These patterns were viewed as pattern alternating and on-/off stimuli. P100/P1 amplitude increased linearly with the relative area white in the pattern, while N135/N1 and P240/P2 amplitude increased with the number of sectors of which the area white was distributed. The decreases P100 amplitude with increasing number of sectors is attributed to an interaction between electric dipoles located in granular and supragranular layers of V1. Differences between the VEP components obtained during a pattern reversing display and following pattern onset are accounted for by the transient and sustained nature of neural responses processing temporal - and spatial luminance contrast and ability of these responses to manifest in the VEP.

From Receptive to Perceptive Fields: Size-Dependent Asymmetries in Both Negative Afterimages and Subcortical On and Off Post-Stimulus Responses

Negative afterimages are perceptual phenomena that occur after physical stimuli disappear from sight. Their origin is linked to transient post-stimulus responses of visual neurons. The receptive fields (RFs) of these subcortical ON- and OFF-center neurons exhibit antagonistic interactions between central and surrounding visual space, resulting in selectivity for stimulus polarity and size. These two features are closely intertwined, yet their relationship to negative afterimage perception remains unknown. Here we tested whether size differentially affects the perception of bright and dark negative afterimages in humans of both sexes, and how this correlates with neural mechanisms in subcortical ON and OFF cells. Psychophysically, we found a size-dependent asymmetry whereby dark disks produce stronger and longer-lasting negative afterimages than bright disks of equal contrast at sizes >0.8°. Neurophysiological recordings from retinal and relay cells in female cat dorsal lateral geniculate nucleus showed that subcortical ON cells exhibited stronger sustained post-stimulus responses to dark disks, than OFF cells to bright disks, at sizes >1°. These sizes agree with the emergence of center-surround antagonism, revealing stronger suppression to opposite-polarity stimuli for OFF versus ON cells, particularly in dorsal lateral geniculate nucleus. Using a network-based retino-geniculate model, we confirmed stronger antagonism and temporal transience for OFF-cell post-stimulus rebound responses. A V1 population model demonstrated that both strength and duration asymmetries can be propagated to downstream cortical areas. Our results demonstrate how size-dependent antagonism impacts both the neuronal post-stimulus response and the resulting afterimage percepts, thereby supporting the idea of perceptual RFs reflecting the underlying neuronal RF organization of single cells.SIGNIFICANCE STATEMENT Visual illusions occur when sensory inputs and perceptual outcomes do not match, and provide a valuable tool to understand transformations from neural to perceptual responses. A classic example are negative afterimages that remain visible after a stimulus is removed from view. Such perceptions are linked to responses in early visual neurons, yet the details remain poorly understood. Combining human psychophysics, neurophysiological recordings in cats and retino-thalamo-cortical computational modeling, our study reveals how stimulus size and the receptive-field structure of subcortical ON and OFF cells contributes to the parallel asymmetries between neural and perceptual responses to bright versus dark afterimages. Thus, this work provides a deeper link from the underlying neural mechanisms to the resultant perceptual outcomes.

Effects of tACS-Like Electrical Stimulation on On-Center Retinal Ganglion Cells: Part I

Purpose

Electrical stimulation of the human central nervous system via surface electrodes has been used for both learning enhancement and the amelioration of neurodegenerative or psychiatric disorders. However, data are sparse on how such electrical stimulation affects neural circuits at the cellular level. This study assessed the effects of tACS-like currents at 10 Hz on On-center retinal ganglion cell responsiveness, using the rabbit retina eyecup preparation as a model for central nervous system effects.

Methods

We made extracellular recordings of light-evoked spike responses in different classes of On-center retinal ganglion cells before, during and after brief applications of 1 microampere alternating currents using single electrodes and microelectrode arrays.

Results

tACS-like currents (tACS) of 1 microampere produced effects on On-center ganglion cell response profiles immediately after initiation or cessation of tACS, without driving phase-locked firing in the absence of light stimuli. tACS affected the initial transient responses to light stimulation for all cells, sustained response components (if any) more strongly for sustained cells, and the center-surround balance more strongly for transient cells.

Conclusion

tACS sculpted light-evoked responses that lasted for one or more hours after cessation of current without, itself, directly inducing significant firing changes. Functionally, tACS effects could result in effects on contrast thresholds for both broad classes of cells, but because tACs differentially affects the center-surround balance of transient On-center cells, there may be greater effects on the spatial resolution and gain. The isolated retina appears to be a useful model to understand tACS actions at the neuronal level.

Refractory density model of cortical direction selectivity: Lagged-nonlagged, transient-sustained, and On-Off thalamic neuron-based mechanisms and intracortical amplification

A biophysically detailed description of the mechanisms of the primary vision is still being developed. We have incorporated a simplified, filter-based description of retino-thalamic visual signal processing into the detailed, conductance-based refractory density description of the neuronal population activity of the primary visual cortex. We compared four mechanisms of the direction selectivity (DS), three of them being based on asymmetrical projections of different types of thalamic neurons to the cortex, distinguishing between (i) lagged and nonlagged, (ii) transient and sustained, and (iii) On and Off neurons. The fourth mechanism implies a lack of subcortical bias and is an epiphenomenon of intracortical interactions between orientation columns. The simulations of the cortical response to moving gratings have verified that first three mechanisms provide DS to an extent compared with experimental data and that the biophysical model realistically reproduces characteristics of the visual cortex activity, such as membrane potential, firing rate, and synaptic conductances. The proposed model reveals the difference between the mechanisms of both the intact and the silenced cortex, favoring the second mechanism. In the fourth case, DS is weaker but significant; it completely vanishes in the silenced cortex.DS in the On-Off mechanism derives from the nonlinear interactions within the orientation map. Results of simulations can help to identify a prevailing mechanism of DS in V1. This is a step towards a comprehensive biophysical modeling of the primary visual system in the frameworks of the population rate coding concept.

A major mechanism that underlies tuning of cortical neurons to the direction of a moving stimulus is still debated. Considering the visual cortex structured with orientation-selective columns, we have realized and compared in our biophysically detailed mathematical model four hypothetical mechanisms of the direction selectivity (DS) known from experiments. The present model accomplishes our previous model that was tuned to experimental data on excitability in slices and reproduces orientation tuning effects in vivo. In simulations, we have found that the convergence of inputs from so-called transient and sustained (or lagged and nonlagged) thalamic neurons in the cortex provides an initial bias for DS, whereas cortical interactions amplify the tuning. In the absence of any bias, DS emerges as an epiphenomenon of the orientation map. In the case of a biased convergence of On- and Off- thalamic inputs, DS emerges with the help of the intracortical interactions on the orientation map, also. Thus, we have proposed a comprehensive description of the primary vision and revealed characteristic features of different mechanisms of DS in the visual cortex with columnar structure.

Optimising the glaucoma signal/noise ratio by mapping changes in spatial summation with area-modulated perimetric stimuli

Identification of glaucomatous damage and progression by perimetry are limited by measurement and response variability. This study tested the hypothesis that the glaucoma damage signal/noise ratio is greater with stimuli varying in area, either solely, or simultaneously with contrast, than with conventional stimuli varying in contrast only (Goldmann III, GIII). Thirty glaucoma patients and 20 age-similar healthy controls were tested with the Method of Constant Stimuli (MOCS). One stimulus modulated in area (A), one modulated in contrast within Ricco’s area (C_R), one modulated in both area and contrast simultaneously (AC), and the reference stimulus was a GIII, modulating in contrast. Stimuli were presented on a common platform with a common scale (energy). A three-stage protocol minimised artefactual MOCS slope bias that can occur due to differences in psychometric function sampling between conditions. Threshold difference from age-matched normal (total deviation), response variability, and signal/noise ratio were compared between stimuli. Total deviation was greater with, and response variability less dependent on defect depth with A, AC, and C_R stimuli, compared with GIII. Both A and AC stimuli showed a significantly greater signal/noise ratio than the GIII, indicating that area-modulated stimuli offer benefits over the GIII for identifying early glaucoma and measuring progression.

GlyRα2, not GlyRα3, modulates the receptive field surround of OFF retinal ganglion cells

Receptive fields (RFs) of most retinal ganglion cells (RGCs) consist of an excitatory center and suppressive surround. The RF center arises from the summation of excitatory bipolar cell glutamatergic inputs, whereas the surround arises from lateral inhibitory inputs. In the retina, both gamma amino butyric acid (GABA) and glycine are inhibitory neurotransmitters. A clear role for GABAergic inhibition modulating the RGC RF surround has been demonstrated across species. Glycinergic inhibition is more commonly associated with RF center modulation, although there is some evidence that it may contribute to the RF surround. The synaptic glycinergic chloride channels are formed by three homomeric β and two homomeric α subunits that can be glycine receptor (GlyR) α1, α2, α3, or α4. GlyRα composition is responsible for currents with distinct decay kinetics. Their expression within the inner plexiform laminae and neuronal subtypes also differ. We studied the role of GlyR subunit selective modulation of RGC RF surrounds, using mice lacking GlyRα2 (Glra2 -/-), GlyRα3 (Glra3 -/-), or both (Glra2/3 -/-). We chose this molecular genetic approach instead of pharmacological manipulation because there are no subunit selective antagonists and strychnine blocks all GlyRs. Comparisons of annulus-evoked responses among wild type (WT) and GlyRα knockouts (Glra2 -/-, Glra3 -/- and Glra2/3 -/-) show that GlyRα2 inhibition enhances RF surround suppression and post-stimulus excitation in only WT OFF RGCs. Similarities in the responses in Glra2 -/- and Glra2/3 -/- RGCs verify these conclusions. Based on previous and current data, we propose that GlyRα2-mediated input uses a crossover inhibitory circuit. Further, we suggest that GlyRα2 modulates the OFF RGC RF center and surround independently. In summary, our results define a selective GlyR subunit-specific control of RF surround suppression in OFF RGCs.

Effects of Auditory Deprivation on Cutaneous Sensitivity

Numerous experiments have shown that visual deprivation (darkness) produces cross-modal sensory enhancement effects. Only recently it has been demonstrated that auditory deprivation can produce similar effects. An experiment was conducted where subjects were exposed to a 1-wk. period of auditory deprivation (silence) with measures of their tactual fusion threshold being taken at daily intervals. The experimental subjects showed, relative to a control condition, a lowering of their threshold. The results are discussed in the context of the sensoristatic model, and the latter's capacity to account for these effects.

Visual Functions of the Thalamus

The thalamus is the heavily interconnected partner of the neocortex. All areas of the neocortex receive afferent input from and send efferent projections to specific thalamic nuclei. Through these connections, the thalamus serves to provide the cortex with sensory input, and to facilitate interareal cortical communication and motor and cognitive functions. In the visual system, the lateral geniculate nucleus (LGN) of the dorsal thalamus is the gateway through which visual information reaches the cerebral cortex. Visual processing in the LGN includes spatial and temporal influences on visual signals that serve to adjust response gain, transform the temporal structure of retinal activity patterns, and increase the signal-to-noise ratio of the retinal signal while preserving its basic content. This review examines recent advances in our understanding of LGN function and circuit organization and places these findings in a historical context.

Visible Persistence of Single-Transient Random Dot Patterns: Spatial Parameters Affect the Duration of Fading Percepts

Visible persistence refers to the continuation of visual perception after the physical termination of a stimulus. We studied an extreme case of visible persistence by presenting two matrices of randomly distributed black and white pixels in succession. On the transition from one matrix to the second, the luminance polarity of all pixels within a disk- or annulus-shaped area reversed, physically creating a single second-order transient signal. This transient signal produces the percept of a disk or an annulus with an abrupt onset and a gradual offset. To study the nature of this fading percept we varied spatial parameters, such as the inner and the outer diameter of annuli (Experiment I) and the radius and eccentricity of disks (Experiment III), and measured the duration of visible persistence by having subjects adjust the synchrony of the onset of a reference stimulus with the onset or the offset of the fading percept. We validated this method by comparing two modalities of the reference stimuli (Experiment I) and by comparing the judgments of fading percepts with the judgments of stimuli that actually fade in luminance contrast (Experiment II). The results show that (i) irrespective of the reference modality, participants are able to precisely judge the on- and the offsets of the fading percepts, (ii) auditory reference stimuli lead to higher visible persistence durations than visual ones, (iii) visible persistence duration increases with the thickness of annuli and the diameter of disks, but decreases with the diameter of annuli, irrespective of stimulus eccentricity. These effects cannot be explained by stimulus energy, which suggests that more complex processing mechanisms are involved. Seemingly contradictory effects of disk and annulus diameter can be unified by assuming an abstract filling-in mechanism that speeds up with the strength of the edge signal and takes more time the larger the stimulus area is.

Representation of central and peripheral vision in the primate cerebral cortex: Insights from studies of the marmoset brain

How the visual field is represented by neurons in the cerebral cortex is one of the most basic questions in visual neuroscience. However, research to date has focused heavily on the small part of the visual field within, and immediately surrounding the fovea. Studies on the cortical representation of the full visual field in the primate brain are still scarce. We have been investigating this issue with electrophysiological and anatomical methods, taking advantage of the small and lissencephalic marmoset brain, which allows easy access to the representation of the full visual field in many cortical areas. This review summarizes our main findings to date, and relates the results to a broader question: is the peripheral visual field processed in a similar manner to the central visual field, but with lower spatial acuity? Given the organization of the visual cortex, the issue can be addressed by asking: (1) Is visual information processed in the same way within a single cortical area? and (2) Are different cortical areas specialized for different parts of the visual field? The electrophysiological data from the primary visual cortex indicate that many aspects of spatiotemporal computation are remarkably similar across the visual field, although subtle variations are detectable. Our anatomical and electrophysiological studies of the extrastriate cortex, on the other hand, suggest that visual processing in the far peripheral visual field is likely to involve a distinct network of specialized cortical areas, located in the depths of the calcarine sulcus and interhemispheric fissure.

Ongoing temporal coding of a stochastic stimulus as a function of intensity: time-intensity trading

Stimulus-locked temporal codes are increasingly seen as relevant to perception. The timing of action potentials typically varies with stimulus intensity, and the invariance of temporal representations with intensity is therefore an issue. We examine the timing of action potentials in cat auditory nerve to broadband noise presented at different intensities, using an analysis inspired by coincidence detection and by the binaural "latency hypothesis." It is known that the two cues for azimuthal sound localization, interaural intensity or level differences and interaural time differences (ITDs), interact perceptually. According to the latency hypothesis, the increase in intensity for the ear nearest to a sound source off the midline causes a decrease in response latency in that ear relative to the other ear. We found that changes in intensity cause small but systematic shifts in the ongoing timing of responses in the auditory nerve, generally but not always resulting in shorter delays between stimulus onset and neural response for increasing intensity. The size of the temporal shifts depends on characteristic frequency with a pattern indicating a fine-structure and an envelope response regime. Overall, the results show that ongoing timing is remarkably stable with intensity at the most peripheral neural level. The results are not consistent in a simple way with the latency hypothesis, but because of the acute sensitivity to ITDs, the subtle effects of intensity on timing may nevertheless have perceptual consequences.

Behavioural assessment of flicker fusion frequency in chicken Gallus gallus domesticus

To interact with its visual environment, an organism needs to perceive objects in both space and time. High temporal resolution is hence important to the fitness of diurnally active animals, not least highly active aerial species such as birds. However, temporal resolution, as assessed by flicker fusion frequency (FFF; the stimulus frequency at which a flickering light stimulus can no longer be resolved and appears continuous) or critical flicker fusion frequency (CFF; the highest flicker fusion frequency at any light intensity) has rarely been assessed in birds. In order to further our understanding of temporal resolution as a function of light intensity in birds we used behavioural experiments with domestic chickens (Gallus gallus domesticus) from an old game breed 'Gammalsvensk dvärghöna' (which is morphologically and behaviourally similar to the wildtype ancestor, the red jungle fowl, G. gallus), to generate an 'Intensity/FFF curve' (I/FFF curve) across full spectrum light intensities ranging from 0.2 to 2812 cd m⁻². The I/FFF curve is double-branched, resembling that of other chordates with a duplex retina of both rods and cones. Assuming that the branches represent rod and cone mediated responses respectively, the break point between them places the transition between scotopic and photopic vision at between 0.8 and 1.9 cd m⁻². Average FFF ranged from 19.8 Hz at the lowest light intensity to a CFF 87.0 Hz at 1375 cd m⁻². FFF dropped slightly at the highest light intensity. There was some individual variation with certain birds displaying CFFs of 90-100 Hz. The FFF values demonstrated by this non-selected breed appear to be considerably higher than other behaviourally derived FFF values for similar stimuli reported for white and brown commercial laying hens, indicating that the domestication process might have influenced temporal resolution in chicken.

Correlation between visual stimulus eccentricity and multiscale neuronal activity in the lateral geniculate nucleus

Single unit activity (SUA) was extensively studied in the lateral geniculate nucleus (LGN) but less attention was paid to the analysis of the local field potentials (LFP). In the present study, we investigate how and to what extent LFP and SUA correlate with visual stimulus eccentricity. SUAs and LFPs recorded extracellularly from 52 electrode positions were analyzed. Both LFP and SUA recordings contained well defined time-segments, which correlated with stimulus eccentricity. The spectral analysis of the LFPs indicated that in addition to the phasic, short latency activity of the 20 Hz frequency band, a tonic, 2-10 Hz, elongated component was also present. The time-domain analysis of the phasic and tonic LFP segments revealed a non-linear decrease of the mean LFP amplitude. The frequency-domain investigation made it obvious that the low and high frequency components exhibit a spatially localized increase of the response, in contrast to the time-domain curve. Our results confirm that the local field potentials as a measure of the mesoscopic level neuronal activity provide additional information concerning the activity of neuronal populations, thus enhancing our present knowledge about the functional circuitry as the foundation of various neuronal processes.

Receptive fields and response properties of neurons in layer 4 of ferret visual cortex

The ferret has become a model animal for studies exploring the development of the visual system. However, little is known about the receptive-field structure and response properties of neurons in the adult visual cortex of the ferret. We performed single-unit recordings from neurons in layer 4 of adult ferret primary visual cortex to determine the receptive-field structure and visual-response properties of individual neurons. In particular, we asked what is the spatiotemporal structure of receptive fields of layer 4 neurons and what is the orientation selectivity of layer 4 neurons? Receptive fields of layer 4 neurons were mapped using a white-noise stimulus; orientation selectivity was determined using drifting, sine-wave gratings. Our results show that most neurons (84%) within layer 4 are simple cells with elongated, spatially segregated, ON and OFF subregions. These neurons are also selective for stimulus orientation; peaks in orientation-tuning curves have, on average, a half-width at half-maximum response of 21.5 +/- 1.2 degrees (mean +/- SD). The remaining neurons in layer 4 (16%) lack orientation selectivity and have center/surround receptive fields. Although the organization of geniculate inputs to layer 4 differs substantially between ferret and cat, our results demonstrate that, like in the cat, most neurons in ferret layer 4 are orientation-selective simple cells.

Measuring and modelling the photopic flicker sensitivity of the chicken (Gallus g. domesticus)

The photopic flicker sensitivity of the chicken was determined using an operant conditioning psychophysical technique. The results show both high- and low-frequency fall-off in the sensitivity response, which peaked around 15 Hz. Flicker sensitivity was determined for a range of stimulus luminance levels, and directly compared to human flicker response measured under similar stimulus conditions. At five luminance levels (10, 100, 200, 500 and 1000 cd/m(2)), the overall chicken flicker sensitivity was found to be considerably lower than for humans, except at high frequencies. A greater degree of frequency tuning was also found in the chicken response. The critical flicker fusion values were either similar or slightly higher for chickens compared to humans (40.8, 50.4, 53.3, 58.2 and 57.4 Hz vs 39.2, 54.0, 54.0, 57.4 and 71.5 Hz respectively for humans and chickens for increasing stimulus luminance level). A recently proposed model for flicker sensitivity [Vision Research 39 (1999) 533], which incorporates low- and high-pass temporal filters in cascade, was found to be applicable to the chicken response. From this model, deductions were made concerning mechanisms controlling the transfer of temporal information.

Visual physiology of the lateral geniculate nucleus in two species of new world monkey: Saimiri sciureus and Aotus trivirgatis

1. Visual responses were recorded from neurones in the magnocellular and parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus in two species of New World monkeys - the diurnal squirrel monkey (Saimiri sciureus) and the nocturnal owl monkey (Aotus trivirgatis). Recording sites were reconstructed in postmortem tissue and comparisons were made between the response properties of magnocellular and parvocellular neurones. 2. Receptive fields were characterized with both white noise and drifting gratings. We found that most of the differences between magnocellular and parvocellular neurones that have been described in the macaque monkey hold for the squirrel monkey and owl monkey. In squirrel monkey and owl monkey, receptive fields of magnocellular neurones were larger than those of parvocellular neurones at similar eccentricities. Although visual responses in the owl monkey were significantly slower than in the squirrel monkey, in both species magnocellular neurones differed from parvocellular neurones in that their responses (1) had higher contrast gains, (2) tended to peak at higher temporal frequencies (but with considerable overlap), (3) had shorter response latencies, and (4) were more transient. 3. The strength of a neurone's receptive-field surround was assessed by comparing neuronal responses to gratings of optimal spatial frequency with responses to gratings of low spatial frequency. Using this approach, receptive-field surrounds were found to be equally strong on average for magnocellular and parvocellular neurones. 4. Spatial summation, as measured by a null test, was linear for all magnocellular and parvocellular cells tested; that is, Y cells were not observed in either species. Finally, most magnocellular neurones showed a contrast gain control mechanism, although this was not seen for parvocellular neurones.

Specificity and strength of retinogeniculate connections

Retinal ganglion cells and their target neurons in the principal layers of the lateral geniculate nucleus (LGN) of the thalamus have very similar, center-surround receptive fields. Although some geniculate neurons are dominated by a single retinal afferent, others receive both strong and weak inputs from several retinal afferents. In the present study, experiments were performed in the cat that examined the specificity and strength of monosynaptic connections between retinal ganglion cells and their target neurons. The responses of 205 pairs of retinal ganglion cells and geniculate neurons with overlapping receptive-field centers or surrounds were studied. Receptive fields were mapped quantitatively using a white-noise stimulus; connectivity was assessed by cross-correlating the retinal and geniculate spike trains. Of the 205 pairs, 12 were determined to have monosynaptic connections. Both the likelihood that cells were connected and the strength of connections increased with increasing similarity between retinal and geniculate receptive fields. Connections were never found between cells with <50% spatial overlap between their centers. The results suggest that although geniculate neurons often receive input from several retinal afferents, these multiple afferents represent a select subset of the retinal ganglion cells with overlapping receptive-field centers.

Effects of attention on orientation-tuning functions of single neurons in macaque cortical area V4

We examined how attention affected the orientation tuning of 262 isolated neurons in extrastriate area V4 and 135 neurons in area V1 of two rhesus monkeys. The animals were trained to perform a delayed match-to-sample task in which oriented stimuli were presented in the receptive field of the neuron being recorded. On some trials the animals were instructed to pay attention to those stimuli, and on other trials they were instructed to pay attention to other stimuli outside the receptive field. In this way, orientation-tuning curves could be constructed from neuronal responses collected in two behavioral states: one in which those stimuli were attended by the animal and one in which those stimuli were ignored by the animal. We fit Gaussians to the neuronal responses to twelve different orientations for each behavioral state. Although attention enhanced the responses of V4 neurons (median 26% increase) and V1 neurons (median 8% increase), selectivity, as measured by the width of its orientation-tuning curve, was not systematically altered by attention. The effects of attention were consistent with a multiplicative scaling of the driven response to all orientations. We also found that attention did not cause systematic changes in the undriven activity of the neurons.

Effects of attention on orientation-tuning functions of single neurons in macaque cortical area V4

We examined how attention affected the orientation tuning of 262 isolated neurons in extrastriate area V4 and 135 neurons in area V1 of two rhesus monkeys. The animals were trained to perform a delayed match-to-sample task in which oriented stimuli were presented in the receptive field of the neuron being recorded. On some trials the animals were instructed to pay attention to those stimuli, and on other trials they were instructed to pay attention to other stimuli outside the receptive field. In this way, orientation-tuning curves could be constructed from neuronal responses collected in two behavioral states: one in which those stimuli were attended by the animal and one in which those stimuli were ignored by the animal. We fit Gaussians to the neuronal responses to twelve different orientations for each behavioral state. Although attention enhanced the responses of V4 neurons (median 26% increase) and V1 neurons (median 8% increase), selectivity, as measured by the width of its orientation-tuning curve, was not systematically altered by attention. The effects of attention were consistent with a multiplicative scaling of the driven response to all orientations. We also found that attention did not cause systematic changes in the undriven activity of the neurons.

The light response of retinal ganglion cells is truncated by a displaced amacrine circuit

The vertebrate retina contains ganglion cells that appear to be specialized for detecting temporal changes. The characteristic response of these cells is a transient burst of action potentials when a stationary image is presented or removed, and often a strong discharge to moving images. These transient and motion-sensitive responses are thought to result from processing in the inner retina that involves amacrine cells, but the critical interactions have been difficult to reveal. Here, we used a cell-ablation technique to remove a subpopulation of amacrine cells from the mouse retina. Their ablation changed transient ganglion cell responses into prolonged discharges. This suggests that transient responses are generated, at least in part, by a truncation of sustained excitatory input to the ganglion cells and that the ablated amacrine cells are critical for this process.

Linear and nonlinear contributions to step responses in cat retinal ganglion cells

We measured excitatory and inhibitory step responses of cat retinal ganglion cells to square wave contrast reversal of stationary sinusoidal gratings. In most Y-cells the initial increase in firing rate (early peak) of the excitatory responses was followed by a distinct second increase in firing rate (late peak). Analysis of the spatial frequency and spatial phase dependence of the two peaks indicated that the early peak appears to be produced by the spatially linear center mechanism, while the late peak appears to be produced by the rectifying subunits described by Hochstein and Shapley (1976) Journal of Physiology, London, 262, 237-264, 265-284. These results indicate that the presence of two peaks in ganglion cell step responses is the result of two excitatory inputs with different time courses, and that inhibitory inputs are not required to explain the appearance of these responses.

Extensive integration field beyond the classical receptive field of cat's striate cortical neurons--classification and tuning properties

Length- and width-summation curves of striate cortex cells revealed that there exist facilitatory, inhibitory or disinhibitory integration fields (IF) beyond the sides and ends of the classical receptive field (RF). The extent of the IFs is most frequently 2-5 times the size of the RFs. The tuning properties of IFs were studied using an annular surround grating patch while an optimal centre patch was placed at the excitatory RF to continuously activate the cell. The results show that, for most cells, the orientation, spatial frequency and speed tuning of the IFs were similar to, but broader than, the tuning of the RF, whereas the direction selectivity of the IF was not as pronounced as that of the RF. The possible functional significance of the IF is discussed.

Cortical feedback increases visual information transmitted by monkey parvocellular lateral geniculate nucleus neurons

We studied the effect of cooling the striate cortex on parvocellular lateral geniculate nucleus (PLGN) neurons in awake monkeys. Cooling the striate cortex produced both facilitation and inhibition of the responses of all neurons, depending on the stimulus presented. Cooling the striate cortex also altered the temporal distribution of spikes in the responses of PLGN neurons. Shannon's information measure revealed that cooling the striate cortex reduced the average stimulus-related information transmitted by all PLGN neurons. The reduction in transmitted information was associated with both facilitation and inhibition of the response. Cooling the striate cortex reduced the amount of information transmitted about all of the stimulus parameters tested: pattern, luminance, spatial contrast, and sequential contrast. The effect of cooling was nearly the same for codes based on the number of spikes in the response as for codes based on their temporal distribution. The reduction in transmitted information occurred because the differences among the responses to different stimuli (signal separation) were reduced, not because the variability of the responses to individual stimuli (noise) was increased. We conclude that one function of corticogeniculate feedback is to improve the ability of PLGN neurons to discriminate among stimuli by enhancing the differences among their responses.

The inverse intensity effect is not lost with stimuli in apparent motion

The inverse relationship between the visible persistence of a briefly presented stimulus and its intensity is well established for static displays. However, with non-static displays, this relationship is only partially reported by previous studies. In order to clarify this topic, we investigated the effect of luminance on the visible persistence of a stimulus in apparent motion. Assuming that persistence duration is a normally distributed random variable, we studied whether the mean persistence of a stimulus could be systematically varied by varying its luminance. Our paradigm permits evaluation of this effect without changing the temporal interval between two successive presentations of the stimulus, thus avoiding the potential influence of this latter factor on persistence. Our results show that the inverse intensity effect still occurs at each of the successive locations of a stimulus in apparent motion. In addition, we provide evidence that increasing the spatial separation between the successive presentations, and decreasing the background luminance, result both in longer persistence duration. Altogether, these findings favour the hypothesis that persistence is actively suppressed by inhibitory interactions between adjacent neural zones.

OFF-alpha and OFF-beta ganglion cells in cat retina: II. Neural circuitry as revealed by electron microscopy of HRP stains

An OFF-center alpha and an OFF-center beta ganglion cell in cat retina, which had been recorded from and intracellularly stained with horseradish peroxidase (HRP) were examined by serial section electron microscopy. We counted synapses and identified presynaptic neurons to the HRP-stained cells in 20 microns radial slices through the centers of their dendritic trees. Presynaptic amacrine and bipolar cells were identified on cytological criteria known from previous studies. The OFF-beta cell with a 62 microns dendritic arbor, restricted to S1 and S2 (sublamina a) of the inner plexiform layer (IPL), received 38% bipolar and 62% amacrine cell synapses. The bipolar input was from both cb1 and cb2 cone bipolar types. Input from three distinct amacrine cell types occurred upon the dendrites, namely from: (1) AII amacrine lobular appendages, (2) large pale amacrine profiles (possibly A2 or A3 cells), and (3) small, dark amacrine types (possibly A8 cells). Large pale amacrine profiles (possibly A13) were found on the cell body and apical dendrite in sublamina b of the IPL. In addition, several amacrine profiles synapsed directly on the sides and base of the cell body in the ganglion cell layer. We estimate that the complete dendritic tree of this beta cell received about 1,000 synapses contributed by 12-14 bipolar cells, 7-10 AII amacrines and 28-41 other amacrine cells. The OFF-alpha cell had a dendritic tree size of 680 x 920 microns. A 250 microns length of two major dendrites stratifying narrowly in S2 of the IPL was reconstructed. Amacrine cells provided most of the synaptic input (80%). This input came from: (1) AII amacrine lobular appendages, (2) amacrines exhibiting large, pale synaptic profiles (possibly A2 or A3 cells), (3) pale amacrines with large mitochondria and a few neurotubules (unknown type), and (4) densely neurotubule-filled amacrine profiles (possibly A19 cells). A large pale amacrine cell type (possibly A13) provided synaptic input to the cell body as a serial synaptic intermediary with rod bipolar cells. Cone bipolar synapses were from only one type of cone bipolar, the cb2 type and formed 20% of the total synaptic input. We estimate that a minimum of 142 bipolar cells, 256 AII amacrine cells and 1,011 other amacrine cells, altogether providing 6,000-10,000 synapses, converged on the dendritic tree of this OFF-alpha cell.

Electrophysiological properties of neurones following mild and acute retinal ischaemia

Early electrophysiological changes following acute retinal ischaemia were studied by recording single or multiunit retinal ganglion cells and the electroretinogram (ERG) in barbiturate anaesthetized cats. Retinal ischaemia was initiated photochemically by platelet aggregation in retinal vessels which had been irradiated with monochromatic green light following an intravenous injection of Rose Bengal dye. No physiologically active ganglion cells were found within, or close to, the irradiated sites with chorioretinal oedema. On the other hand, in the areas 5-20 degrees away from the irradiation sites, ganglion cells had abnormally raised spontaneous (background) firing which obscured visually driven firing. The retinal areas where no physiologically active ganglion cells were found showed histopathological changes which are similar to those described for glutamate-induced retinal damage. Retinal areas where depolarized retinal ganglion cells were located, however, showed only minor vacuolation of the ganglion cell fibre layer. Early global electrophysiological changes following photochemically induced retinal vascular lesion were consistent with those predicted from the findings in the single cell study. Vascular lesions produced with high irradiation energy (10-30 J), which promote extensive chorioretinal oedema, resulted in gradual loss of visually responsive ganglion cells. Lesions produced by low-energy irradiation (2 J), causing slight narrowing of the blood columns in the vessels, on the other hand, resulted in significant increases in the amplitude and the implicit time of the ERG b-wave and the background firing of multiunit retinal ganglion cells. Electrophysiological changes associated with mild retinal ischaemia are analogous to physiological effects associated with exogenous glutamate or blockade of glutamate uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Deficits in initial feature registration of schizophrenics and substance abusers

Information processing deficits are consistently reported for schizophrenics. The present study evaluated if the longer duration required by schizophrenics to visually recognize a target may qualify, as proposed by previous studies, as a vulnerability and/or trait biological marker. Critical stimulus duration (CSD) was used as the index of initial target registration and recognition. The CSD is the minimal duration, in ms, to meet task criterion, which in the present study was seven consecutive identifications of the target letter 'T' or 'A'. There were 13 normal controls, 11 methadone maintenance experimental controls, 21 chronic schizophrenics and 12 subacute schizophrenics. Analysis of variance revealed that the CSDs of normal controls and subacute schizophrenics were not statistically different (p > 0.05); the CSDs of chronic schizophrenics were not statistically different from methadone controls (p > 0.05), while the chronic schizophrenics and the methadone controls' CSDs were statistically different from the normal controls and the subacute schizophrenics. The results support earlier reports of long target duration required by chronic schizophrenics for feature recognition. Since retarded CSDs were obtained for methadone control but not for acute schizophrenics, the CSD does not qualify as a specificity or a vulnerability index for schizophrenia. A neurophysiological explanation is proposed for the findings.

Does attention modulate the perception of luminance changes?

In a previous study assessing the modulation of visual processing by attention, Bonnel, Possamî, and Schmitt showed that, when discriminating line-length, subjects precisely shared processing resources between two pairs of lines presented to the left and right of fixation. In a close replication requiring the detection of luminance increments instead of line-length differences, subjects were unable to follow the instructions and to allocate attention differentially, thus supporting the claim that light detection is fundamentally different from shape discrimination. In a subsequent experiment, we tested and rejected the possibility that luminance perception was not open to modulation by attention due to its physical nature. Replacing brightness detection by brightness identification allowed voluntary control on the quality of processing to be evidenced. The similarity between the latter results and the data from line-length discrimination suggests that task requirements may be crucial in determining the distribution of attention.

Extensive disinhibitory region beyond the classical receptive field of cat retinal ganglion cells

Forty-three single fibres were recorded from cat optic tract. They were tested with flashing lines of different lengths and/or spots of different dimensions. The area of centre excitation and of surround inhibition were represented in the length (or dimension)-response functions by the steep rising and falling phases, respectively. For most of the cells, a secondary gentle rise was seen when the stimulus extended beyond the inhibitory surround, indicating an extensive disinhibitory region (DIR) outside the classical receptive field (RF). Disinhibition appeared and developed concurrently with surround inhibition at stimulus intensities between 3 and 30 cd/m2, being represented in the phasic or the tonic component of responses depending on the cell type recorded.

Spontaneous firing level distinguishes the effects of NMDA and non-NMDA receptor antagonists on the ganglion cells in the cat retina

Different responses of retinal ganglion cells to iontophoretically applied NMDA receptor antagonists and non-NMDA receptor antagonists were studied in anaesthetized cats. Cells with normal range of spontaneous firing and those with abnormally high spontaneous firing levels showed a different response to these drugs. Both visually driven and spontaneous firing of cells with 'normal' spontaneous firing level were blocked by non-NMDA receptor antagonists, but not by NMDA receptor antagonists which often raised spontaneous firing. In contrast, the responses of cells with abnormally high spontaneous firing level were blocked effectively by NMDA antagonists including MK-801, an NMDA channel blocker, as well as by non-NMDA receptor antagonists. The results suggest that under normal physiological conditions, NMDA receptors which are not involved in synaptic transmission may play a role in reducing the resting discharge level of the retinal ganglion cells. NMDA receptors, however, appear to open ion channels in response to glutamate input when ganglion cells become abnormally depolarized.

Evidence for independent processing of subjective contour brightness and sharpness

Subjective contours have been of considerable interest because of their importance to theories and physiological models of form perception. In particular, they have recently been characterized as the result of magnocellular cortical processing. There is, however, a paucity of parametric data relating to basic psychophysical parameters in this field. Two experiments are reported in which the roles of subjective contour size, retinal eccentricity, and flicker rate in subjective contour salience were investigated. Eleven observers estimated subjective contour magnitude using an Ehrenstein configuration. Configurations ranging in size from 0.25 to 3 deg were presented to three retinal loci (fovea, 2 deg, and 4 deg) at flicker rates ranging from 5 to 15 Hz. Subjective contour brightness and distinctness were measured separately. Brightness was greatest at a subjective contour size of about 1.25 deg, at flicker rates of 5-7 Hz, and at 3 deg peripheral for all flicker rates and all but the smallest stimulus sizes. Distinctness decreased with eccentricity and flicker, but remained high at small diameters (thus implicating spatially sensitive mechanisms). Taken together, the results support a magnocellular processing of subjective contours with respect to brightness, but also suggest that there is a parvocellular contribution to subjective contour sharpness.

Identification of X versus Y properties for interneurons in the A-laminae of the cat's lateral geniculate nucleus

Roughly 25% of the neurons in the A-laminae of the cat's lateral geniculate nucleus are local interneurons, while the remaining 75% are relay cells that project to the visual cortex. The interneurons form the focus of our study. The relay cells are either X or Y cells and are thereby integral links in the parallel and independent retino-geniculo-cortical X and Y pathways. Little is known about the response properties of interneurons, largely because it is difficult to identify them clearly during electrophysiological recording. However, they can be identified by morphological criteria. We thus studied their response properties by recording intracellularly from geniculate neurons to characterize them and then injecting them with horseradish peroxidase (HRP); the HRP labeling subsequently allowed us to distinguish relay cells from interneurons. In this manner, we studied 171 relay cells (83 X and 88 Y) and 15 interneurons. The response properties tested for each of the interneurons were indistinguishable from those of the relay X cells. We conclude that these interneurons are directly innervated by retinogeniculate X axons and are firmly embedded in the X pathway. We found no evidence for interneurons in the Y pathway.