Disruptions to human speed perception induced by motion adaptation and transcranial magnetic stimulation.

To investigate the underlying nature of the effects of transcranial magnetic stimulation (TMS) on speed perception, we applied repetitive TMS (rTMS) to human V5/MT+ following adaptation to either fast- (20 deg/s) or slow (4 deg/s)-moving grating stimuli. The adapting stimuli induced changes in the perceived speed of a standard reference stimulus moving at 10 deg/s. In the absence of rTMS, adaptation to the slower stimulus led to an increase in perceived speed of the reference, whilst adaptation to the faster stimulus produced a reduction in perceived speed. These induced changes in speed perception can be modelled by a ratio-taking operation of the outputs of two temporally tuned mechanisms that decay exponentially over time. When rTMS was applied to V5/MT+ following adaptation, the perceived speed of the reference stimulus was reduced, irrespective of whether adaptation had been to the faster- or slower-moving stimulus. The fact that rTMS after adaptation always reduces perceived speed, independent of which temporal mechanism has undergone adaptation, suggests that rTMS does not selectively facilitate activity of adapted neurons but instead leads to suppression of neural function. The results highlight the fact that potentially different effects are generated by TMS on adapted neuronal populations depending upon whether or not they are responding to visual stimuli.

Speed selectivity in visual short term memory for motion.

In this study we employed a 'memory masking' paradigm to determine which stimulus attributes are important in the storage of information about the speed of moving grating stimuli in visual short term memory (VSTM). Delayed speed discrimination thresholds were measured in the presence of masking stimuli which varied in terms of their spatial and temporal frequency content. Memory masking results demonstrate that it is genuinely the speed of the stimulus, as opposed to temporal or spatial frequency content, that is crucial in the retention of information about motion in visual short term memory. The property of speed selectivity exhibited by VSTM mirrors that reported for neurons in area V5/MT, a brain area crucial for the processing of visual motion in primate brain. This link between area V5/MT and VSTM for motion is consistent with current views which suggest that there is a close association between the neural mechanisms involved in the analysis of sensory information and those involved in its retention in short term memory.

Chromatic adaptation, perceived location, and color tuning properties.

We have studied the influence of chromatic adaptation upon the perceived visual position of a test stimulus using a Vernier alignment task. Maximum and minimum offsets in spatial position are generated when the adapting and test stimuli lie on the same and orthogonal axes in MBDKL color space, respectively. When the test stimuli lie on intermediate color axes, the measured positional shifts decrease as a function of the angular separation in color space (phi) from the adapting stimulus. At low stimulus contrasts, these shifts follow a sinusoidal function of phi and exhibit broad chromatic tuning and can be accounted for by a model in which the centroid is extracted from the linear combination of after-image, formed by the adapting stimulus, and the test stimulus. Such linear, broadband behavior is consistent with the response properties of chromatic neurons in the precortical visual pathway. At high contrast, and when adaptation gets closer to the S/(L+M) axis, the tuning functions become narrower and require sinusoids raised to increasingly higher exponents in order to describe the data. This narrowing of chromatic tuning is consistent with the tuning properties of chromatic neurons in the striate cortex, and implies the operation of a nonlinear mechanism in the combination of cone outputs.