The influence of stimulus chromaticity on the isoluminant motion-onset VEP

AbstractMotion-onset visual evoked potentials (VEPs) were elicited by low spatial frequency chromatic isoluminant gratings presented in a central 7° circular field. The chromatic composition of the stimuli was varied so as to modulate along different axes in colour space. For slow speeds (<5°/s) changing the chromatic axis induced large response differences between the S- and L/M-cone VEPs. At faster speeds (5–12°/s) the effects were not as marked. A dichotomy between the slow and fast responses was also shown to exist in terms of their contrast dependencies, the former exhibiting a stronger dependency on contrast than the latter. These findings suggest that neural substrates with chromatic sensitivity are involved in the generation of S- and L/M-cone mediated motion-onset VEPs at low velocities. At higher velocities, responses are generated by different mechanisms that possess little or no chromatic sensitivity

The Cortical Topography of Visual Evoked Potentials Elicited by Chromatic and Luminance Motion

When motion-onset VEPs are elicited by moving luminance patterns, the motion specific component of the response, N2, is more prominent at electrode sites that overlay the lateral occipito-parietal cortex close to area V5/MT, than over the primary visual cortex. Functional segregation suggests that colour and motion processing should take place along different ventral occipito-temporal and lateral occipito-parietal pathways, respectively. Hence, a different topographical distribution might be expected for the motion-onset VEPs elicited by chromatic and luminance motion stimuli. We recorded motion-onset VEPs to moving luminance or isoluminant chromatic sinusoidal grating stimuli from five electrodes sites located at Oz, and at four locations (T1-T4) lateral to Oz, at intervals of 5% of the head circumference. Responses were recorded from 6 subjects over a range of speeds and contrasts. The results showed that the N2 component was maximal at similar lateral electrode locations (T2) for both luminance-defined and chromatically-defined motion. The earlier P1 component was of greatest magnitude at the occipital pole (Oz) and decreased with more lateral electrode placement and again this was the same for colour and luminance responses. These similarities suggest a common origin for VEPs elicited by colour and luminance defined motion

Visual evoked potentials elicited by chromatic motion onset

AbstractVisually Evoked Potentials (VEPs) were recorded in response to the onset of chromatic and luminance motion gratings of 1 cpd and luminance 40 cd m−2 subtending a 7° field. At slow speeds (≤2 cycles s−1) the motion onset response exhibits a clear amplitude minimum at isoluminance. Over the Michelson contrast range tested (0.05–0.75) the chromatic response at 2 cycles s−1 possesses a linear response function compared to the saturating function of the luminance response and the contrast dependency of the former is a factor of 5–6 times greater than for the latter. These differences are suggestive of different neural substrates for the chromatic and luminance motion VEPs at slow speeds. At 10 cycles s−1 the chromatic motion onset VEP exhibits no amplitude minimum at isoluminance and becomes more like its luminance counterpart in terms of its saturating contrast response function. Furthermore, the contrast dependency of the chromatic and luminance responses differs by only a factor of 1.6 at this faster rate. These findings are consistent with the idea of separate motion mechanisms that operate at fast and slow speeds, the latter having separate channels for colour and luminance motion

The contribution of the right supra-marginal gyrus to sequence learning in eye movements.

We investigated the role of the human right Supra-Marginal Gyrus (SMG) in the generation of learned eye movement sequences. Using MRI-guided transcranial magnetic stimulation (TMS) we disrupted neural activity in the SMG whilst human observers performed saccadic eye movements to multiple presentations of either predictable or random target sequences. For the predictable sequences we observed shorter saccadic latencies from the second presentation of the sequence. However, these anticipatory improvements in performance were significantly reduced when TMS was delivered to the right SMG during the inter-trial retention periods. No deficits were induced when TMS was delivered concurrently with the onset of the target visual stimuli. For the random version of the task, neither delivery of TMS to the SMG during the inter-trial period nor during the presentation of the target visual stimuli produced any deficit in performance that was significantly different from the no-TMS or control conditions. These findings demonstrate that neural activity within the right SMG is causally linked to the ability to perform short latency predictive saccades resulting from sequence learning. We conclude that neural activity in rSMG constitutes an instruction set with spatial and temporal directives that are retained and subsequently released for predictive motor planning and responses