Ilya Khaytin

Unequal representation of cardinal vs. oblique orientations in the middle temporal visual area.

A possible neurobiological basis for the “oblique effect” is linked to the finding that more neural machinery is devoted to processing cardinal vs. oblique orientations in primary visual cortex (V1). We used optical imaging to determine whether more territory is devoted to processing horizontal and vertical orientations than oblique orientations in owl monkey middle temporal visual area (MT), a visual area highly sensitive to moving stimuli. We found that more of MT was devoted to representing cardinal than oblique orientations, and that the anisotropy was more prominent in parts of MT representing central vision (≤10°). Neural responses to orientations of 0° and 90° were also greater than those to 45° and 135°. In comparison, an overrepresentation of cardinal orientations in the representation of central vision in owl monkey V1 was relatively small and inconsistent. Our data could explain the greater sensitivity to motion discrimination when stimuli are moved along cardinal meridians and suggest that the neural machinery necessary to explain the motion oblique effect either originates in MT or is enhanced at this level.

The Evolution of Parallel Visual Pathways in the Brains of Primates

Vision is the dominant sense in primates. This article attempts to reconstruct a reasonable scenario as to how parallel visual pathways might have evolved in primates by comparing key factors that might distinguish this group evolutionarily. The focus is on visual information channels from the eye through the lateral geniculate nucleus (LGN) of the thalamus to cortex since these pathways may have become uniquely specialized in primate evolution. We defend the position that magnocellular (M) and parvocellular (P) retinogeniculocortical pathways are homologous across primates and therefore probably existed in the mammalian common ancestor of primates. Whether homologues to these visual pathways can be found in extant mammals remains controversial, but evidence suggests that functionally similar pathways can be identified in a range of mammals. Even for the less well-researched koniocellular (K) pathway, data exist suggesting an early evolutionary history. Only among primates, however, is the evidence strong enough to support homology. We also present data suggesting that the common ancestor to primates was dichromatic and that early primates may even have been diurnal given the existence of genes for at least two cone types in all primates. We also review evidence for homologies between ocular dominance pathways and other properties. In addition, we review evidence for the evolutionary history of cortical hierarchies of visual areas and conclude that only a few areas can be considered homologous across primates and even fewer across mammals. In the final section, we provide a summary and also outline questions that should be addressed in order to arrive at more definitive conclusions concerning the evolution of parallel visual pathways. We also outline some practical strategies for answering some of these questions.

Quantification of Optical Images of Cortical Responses for Inferring Functional Maps

Optical imaging of cortical signals enables the mapping of functional organization across large patches of cortex with good spatial resolution. But techniques for the quantitative analysis and interpretation of these images are limited. Frequently the functional architecture of the cortex is inferred from the visible topography of cortical reflectance images averaged or differenced across stimulus conditions and scaled or color-coded for presentation. Such qualitative assessments have sometimes led to divergent conclusions particularly about the organization of spatial and temporal frequency preferences in the primary visual cortex. We applied quantitative methods derived from signal detection theory to objectively interpret optical images. The differential response to any two arbitrary stimuli was represented at each pixel as the probability of discriminating between the two stimuli given the reflectance values at that pixel. These probability maps reduced false alarms and provided better signal-to-noise ratio in fewer trials than difference maps. We applied these methods to optical images of primate primary visual area (V1) obtained in response to sinusoidal gratings of different orientations and spatiotemporal frequencies. Clustering by orientation preference was stronger than that for spatial frequency, whereas clustering by temporal frequency preference was the weakest, largely in agreement with a previous electrophysiological study that quantified the degree of clustering of neurons for various response properties using uniform, quantitative criterion. We suggest that probability maps can extend the applicability of optical imaging to investigations of finer aspects of cortical functional organization through better signal-to-noise ratio and uniform, quantitative criteria for interpretation.