Macaque Middle Temporal Neurons Signal Depth in the Absence of Motion

Abstract

The middle temporal (MT) visual area is widely accepted to play important roles in motion processing. It is unclear, however, whether MT contributes to visual perception during the viewing of static scenes, when there is little retinal image motion during the interval between saccades. Some previous studies suggest that MT neurons give little or no response to stationary stimuli that are flashed onto the receptive field, but no previous study has directly examined the fidelity with which MT neurons code visual information in moving versus stationary stimuli. In this study, we compare the ability of MT neurons to signal binocular disparity in moving versus stationary random-dot stereograms. Although responses to moving stimuli are typically stronger, many MT neurons give robust responses to stationary stereograms, and some MT neurons actually prefer stationary patterns to those moving at any tested speed. These responses to stationary stimuli are not caused by monitor refresh or microsaccades. Disparity tuning curves for moving and stationary stimuli are nearly identical in shape for most neurons. Although the disparity discriminability of MT neurons is generally higher for moving stereograms when responses are averaged over the entire 1.5 sec trial epoch, discriminability is comparable for moving and stationary stimuli during the first 200 -300 msec of the response. Thus, in a normal time interval between saccades, MT neurons signal the binocular disparity of stationary stimuli with high fidelity. These findings show that MT can be a reliable source of visual information during the viewing of static scenes.

A Logarithmic, Scale-Invariant Representation of Speed in Macaque Middle Temporal Area Accounts for Speed Discrimination Performance

“…There were no specific criteria for including a neuron in this sample; we recorded from every neuron that we could activate with random-dot stimuli over the range of speeds that we could test (in searching for neurons, speeds up to ϳ50°/s were commonly tested). We may not have recorded from neurons that only responded to very fast speeds, but we estimate that, at most, 10 -20% of neurons were passed over on these grounds (Palanca and DeAngelis, 2003).…”

“…Parameters R 0 and A determine the baseline firing rate and depth of modulation, respectively, whereas ␣, , and n determine the shape and location of the function. As the exponent n increases, the Gamma function takes on a range of shapes from exponential to log-Gaussian to Gaussian, making it very flexible for capturing the diversity in shapes of speed-tuning functions that we observed (DeAngelis and Palanca and DeAngelis, 2003). The solid curve in Figure 3A shows the fit of a Gamma function to the speed-tuning curve of an example MT neuron.…”

“…The insets in Figure 5 show curve fits for three neurons that span the range of the data. To summarize the effect of fixing and s 0 , we compared the errors associated with the two log-Gaussian models using a sequential F test (Draper and Smith, 1966;Prince et al, 2002b;Palanca and DeAngelis, 2003), which accounts for the difference in degrees of freedom between the two models. Using this test, we can determine whether each MT neuron was signifi- cantly better fit by the free log-Gaussian model than by the constrained model.…”

“…Using this test, we can determine whether each MT neuron was signifi- cantly better fit by the free log-Gaussian model than by the constrained model. Because the sequential F test is strictly applicable only to functions that are linear in their parameters, we performed Monte Carlo simulations to confirm that our computed F values followed an F distribution with the appropriate degrees of freedom, such that the test had an appropriate rejection rate (Prince et al, 2002b;Palanca and DeAngelis, 2003).…”

MT+/V5 Activation without Conscious Motion Perception: A High-Field fMRI Study

“…MT＋W V5 neurons in human, 5,9 as well as MT neurons in monkeys, 19 are sensitive to ‰ickers and luminance changes even if the stimuli are stationary in space. Temporal changes in luminance at a certain location in the visualˆeld may be indicative of displacement of an object into or out of that speciˆc location.…”