Information processing in the primate visual system

Contribution of parasol-magnocellular pathway ganglion cells to foveal retina in macaque monkey

Parasol-magnocellular pathway ganglion cells form an important output stream of the primate retina and make a major contribution to visual motion detection. They are known to comprise ON and OFF type response polarities but the relative numbers of ON and OFF parasol cells, and the overall contribution of parasol cells to high-acuity foveal vision are not well understood. Here we use antibodies against carbonic anhydrase 8 (CA8) and intracellular injections of the liphilic dye DiI to show that CA8 selectively labels OFF parasol cells in macaque retina. By combined labeling with CA8 antibodies and a previously-described marker for parasol cells (GABA_A receptor antibodies), we show that ON and OFF parasol cells each comprise ∼ 6% of all ganglion cells in central retina (each peak density ∼ 3000 cells/mm² at 5 deg.), and each population comprises ∼ 10% of all ganglion cells in peripheral temporal retina. Thus, the spatial density of parasol cells in central retina is greater than reported by previous anatomical studies, and the central-peripheral gradient in parasol cell density is shallower than previously reported. The data nevertheless predict decline in spatial acuity with visual field eccentricity for both midget-parvocellular pathway and parasol-magnocellular pathway mediated visual functions. The spatial resolving power of the OFF parasol array (peak ∼ 7 cpd) falls short of macaque behavioral grating acuity by at least a factor of three throughout the retina.

New Features of Receptive Fields in Mouse Retina through Spike-triggered Covariance

Retinal ganglion cells (RGCs) encode various spatiotemporal features of visual information into spiking patterns. The receptive field (RF) of each RGC is usually calculated by spike-triggered average (STA), which is fast and easy to understand, but limited to simple and unimodal RFs. As an alternative, spike-triggered covariance (STC) has been proposed to characterize more complex patterns in RFs. This study compares STA and STC for the characterization of RFs and demonstrates that STC has an advantage over STA for identifying novel spatiotemporal features of RFs in mouse RGCs. We first classified mouse RGCs into ON, OFF, and ON/OFF cells according to their response to full-field light stimulus, and then investigated the spatiotemporal patterns of RFs with random checkerboard stimulation, using both STA and STC analysis. We propose five sub-types (T1–T5) in the STC of mouse RGCs together with their physiological implications. In particular, the relatively slow biphasic pattern (T1) could be related to excitatory inputs from bipolar cells. The transient biphasic pattern (T2) allows one to characterize complex patterns in RFs of ON/OFF cells. The other patterns (T3–T5), which are contrasting, alternating, and monophasic patterns, could be related to inhibitory inputs from amacrine cells. Thus, combining STA and STC and considering the proposed sub-types unveil novel characteristics of RFs in the mouse retina and offer a more holistic understanding of the neural coding mechanisms of mouse RGCs.

Temporal processing as a source of altered visual perception in high autistic tendency

Superior local at the expense of global perception characterises vision in autism spectrum disorders (ASD). However, progress towards discovering a neural mechanism has been slow. Here we used known differences in magnocellular and parvocellular receptive field properties to assess the temporal encoding of information, via flicker fusion paradigms, in those high and low in self-reported autistic tendency (Autism Spectrum Quotient - AQ). A Low AQ group (AQ≤13, n=22), and a High AQ group (AQ≥18, n=17) undertook a 4AFC luminance flicker fusion (FF) with 5 temporal contrasts from 5% to 100%, and a 2AFC isoluminant red-green colour fusion task. Both groups showed an increase in fusion thresholds with temporal achromatic contrast. The High AQ group displayed diminished flicker fusion thresholds compared to the Low AQ at the lowest contrasts. For the red-green colour fusion task, the High AQ group displayed mean fusion frequency slightly greater than the Low AQ group. A significant interaction between 5% luminance contrast and the red-green fusion frequencies demonstrated that the differences in thresholds were not simply due to variations in overall attentional capacity between groups. These differences in flicker fusion thresholds are in accordance with reported differences in cortical visual evoked potential nonlinearities, particularly relating to the neural efficiency of the magnocellular pathway.