Biomedical subjects

Jason D Forte

Publications and source records attributed to Jason D Forte.

5 recordsLinked to original sources

PubMed2006

Contribution of chromatic aberrations to color signals in the primate visual system.

We measured responses to red-green color variation in parvocellular (PC) neurons in the lateral geniculate nucleus of dichromatic ("red-green color blind") marmoset monkeys. Although these animals lack distinct visual pigments to distinguish between wavelengths in this range, many of the colored stimuli nevertheless produced robust responses in PC cells. We show that these responses, which are restricted to high stimulus spatial frequencies (fine image details), arise from chromatic aberrations in the eye. The neural signals produced by chromatic aberrations are of comparable magnitude to signals produced by high-frequency luminance (LUM) modulation and thus could influence cortical pathways for processing of color and object recognition. The fact that genetically "color-blind" primates are not necessarily blind to wavelength-dependent contours in the visual world may have enabled red-green color vision to become linked with high-acuity spatial vision during primate evolution.

Animals↗

PubMed2005

Inter-ocular transfer of the tilt illusion shows that monocular orientation mechanisms are colour selective.

A vertical grating appears tilted when surrounded by a tilted inducer grating: the tilt illusion. We investigated the inter-ocular transfer of the tilt illusion for gratings modulated along parallel or orthogonal vectors in a L-M and L+M+S cone contrast space. We found that the monocular component of the tilt illusion is entirely colour selective and the binocular component shows only weak colour selectivity. These results suggest that colour and orientation processing interact at monocular stages of visual processing, whereas binocular visual mechanisms code for form in a manner that is largely insensitive to chromatic signature.

Color Perception↗

PubMed2003

Colour and luminance selectivity of spatial and temporal interactions in orientation perception.

Previous studies have commented upon the similar phenomenology of simultaneous and successive interactions in the perception of orientation. These similarities have been taken as evidence of common mechanisms underlying the simultaneous tilt illusion (TI) and the successive tilt aftereffect (TAE). We measured the TI and TAE for four subjects for combinations of test and inducing stimuli modulated along either the same or orthogonal axes of colour space within the L+M+S, L-M colour-luminance plane. The largest TI and TAE were found when test and inducer were modulated along the same axis of colour space. The TI consistently showed greater selectivity for colour/luminance than the TAE. The results are discussed in relation to the known chromatic properties of the primate visual pathways. Specifically, we suggest that both the TI and TAE involve colour- and luminance-specific neurons in primary visual cortex as well as cue-invariant mechanisms in extrastriate cortex.

Color Perception↗

PubMed2002

Influence of chromaticity on vernier and stereo acuity.

Vernier offset thresholds for targets modulated in luminance or isoluminantly along the L-M axis were confirmed to be equal for targets whose contrasts were equal multiples of those required for detection. On the other hand, stereoscopic depth thresholds were elevated by a factor of 10 or more for isoluminantly modulated targets. Thresholds for vernier targets are 2 or 3 times larger with a gap of 20 arcmin than for a gap of 1 arcmin for both isoluminant and luminance targets. On the other hand, stereo thresholds decrease by a factor of 2 to 3 for both classes of target over the same range. We consider our results in the light of recent electrophysiological and psychophysical evidence and conclude that our results are consistent with the notion that stereo thresholds are mediated by a single class of mechanism for targets modulated in luminance or isoluminantly. We test and reject the hypothesis that stereopsis is subserved by independent chromatic and luminance mechanisms.

Color Perception↗

PubMedDate not supplied

Spatial coding and response redundancy in parallel visual pathways of the marmoset Callithrix jacchus.

Many neurons in the primary visual cortex (area V1) show pronounced selectivity for the orientation and spatial frequency of visual stimuli, whereas most neurons in subcortical afferent streams show little selectivity for these stimulus attributes. It has been suggested that this transformation is a functional sign of increased coding efficiency, whereby the redundancy (or overlap in response properties) is reduced at consecutive levels of visual processing. Here we compared experimentally the response redundancy in area V1 with that in the three main dorsal thalamic afferent streams, the parvocellular (PC), koniocellular (KC), and magnocellular (MC) divisions of the dorsal lateral geniculate nucleus (LGN) in marmosets. The spatial frequency and orientation tuning of single cells in the LGN and area V1 were measured, using luminance contrast sine-wave gratings. A joint spatial frequency-orientation response selectivity profile was calculated for each cell. Response redundancy for each population was estimated by cross-multiplication of the joint selectivity profiles for pairs of cells. We show that when estimated in this way, redundancy in LGN neurons is approximately double that of neurons in cortical area V1. However, there are differences between LGN subdivisions, such that the KC pathway has a spatial representation that lies between the redundant code of the PC and MC pathways and the more efficient sparse spatial code of area V1.

Action Potentials↗