Sensitivity for reverse-phi motion

AbstractLow-level contrast information in the primary visual pathway is represented in two different channels. ON-center cells signal positive contrasts and OFF-center cells signal negative contrasts. In this study we address the question whether initial motion analysis is performed separately in these two channels, or also through combination of signals from ON and OFF cells. We quantitatively compared motion coherence detection for regular and for reverse-phi motion stimuli. In reverse-phi motion the contrast of a pattern flips during displacements. Sensitivity is therefore based on correlating positive and negative contrasts, whereas for regular motion it is based on correlating similar contrasts. We compared tuning curves for step size and temporal interval for stimuli in which motion information was limited to a single combination of step size and interval. Tuning for step size and temporal interval was highly similar for the two types of motion. Moreover, minimal coherence thresholds for both types of motion matched quantitatively, irrespective of dot density. We also measured sensitivity for so-called no-phi motion stimuli, in which the contrast of displaced dots was set to zero. Sensitivity for no-phi motion was low for stimuli containing only black or only white dots. When both dot polarities were present in the stimulus, sensitivity was absent. Thus, motion information based on separate contrasts was effectively cancelled by a component based on different contrasts. Together these results show equal efficiency in correlating dots of opposite contrast and of similar contrast, which strongly suggests efficient detection of correlations across ON and OFF channels

Tuning for temporal interval in human apparent motion detection

Detection of apparent motion in random dot patterns requires correlation across time and space. It has been difficult to study the temporal requirements for the correlation step because motion detection also depends on temporal filtering preceding correlation and on integration at the next levels. To specifically study tuning for temporal interval in the correlation step, we performed an experiment in which prefiltering and postintegration were held constant and in which we used a motion stimulus containing coherent motion for a single interval value only. The stimulus consisted of a sparse random dot pattern in which each dot was presented in two frames only, separated by a specified interval. On each frame, half of the dots were refreshed and the other half was a displaced reincamation of the pattern generated one or several frames earlier. Motion energy statistics in such a stimulus do not vary from frame to frame, and the directional bias in spatiotemporal correlations is similar for different interval settings. We measured coherence thresholds for left-right direction discrimination by varying motion coherence levels in a Quest staircase procedure, as a function of both step size and interval. Results show that highest sensitivity was found for an interval of 17-42 ms, irrespective of viewing distance. The falloff at longer intervals was much sharper than previously described. Tuning for temporal interval was largely, but not completely, independent of step size. The optimal temporal interval slightly decreased with increasing step size. Similarly, the optimal step size decreased with increasing temporal interval.</p

The parallel between reverse-phi and motion aftereffects

Periodically flipping the contrast of a moving pattern causes a reversal of the perceived direction of motion. This direction reversal, known as reverse-phi motion, has been generally explained with the notion that flipping contrasts actually shifted the balance of motion energy toward the opposite direction. In this sense, the reversal is trivial because any suitable motion energy detector would be optimally excited in a direction opposite to that for regular motion. This notion, however, does not address the question how these two types of motion are initially detected. Here we show several perceptual phenomena indicating that low-level detection of the two types of motion is quite different. Reverse-phi motion percepts in many respects behave more like motion aftereffects than like regular motion. Motion adaptation causes reduced activity during a stationary test stimulus, which by means of directional opponency leads to motion perceived in the opposite direction. Our findings suggest that reverse-phi motion similarly reduces the activity of low-level motion detectors.</p

Temporal dynamics of direction tuning in motion-sensitive macaque area MT

We studied the temporal dynamics of motion direction sensitivity in macaque area MT using a motion reverse correlation paradigm. Stimuli consisted of a random sequence of motion steps in eight different directions. Cross-correlating the stimulus with the resulting neural activity reveals the temporal dynamics of direction selectivity. The temporal dynamics of direction selectivity at the preferred speed showed two phases along the time axis: one phase corresponding to an increase in probability for the preferred direction at short latencies and a second phase corresponding to a decrease in probability for the preferred direction at longer latencies. The strength of this biphasic behavior varied between neurons from weak to very strong and was uniformly distributed. Strong biphasic behavior suggests optimal responses for motion steps in the antipreferred direction followed by a motion step in the preferred direction. Correlating spikes to combinations of motion directions corroborates this distinction. The optimal combination for weakly biphasic cells consists of successive steps in the preferred direction, whereas for strongly biphasic cells, it is a reversal of directions. Comparing reverse correlograms to combinations of stimuli to predictions based on correlograms for individual directions revealed several nonlinear effects. Correlations for successive presentations of preferred directions were smaller than predicted, which could be explained by a static nonlinearity (saturation). Correlations to pairs of (nearly) opposite directions were larger than predicted. These results show that MT neurons are generally more responsive when sudden changes in motion directions occur, irrespective of the preferred direction of the neurons. The latter nonlinearities cannot be explained by a simple static nonlinearity at the output of the neuron, but most likely reflect network interactions.</p

Dynamics of directional selectivity in MT receptive field centre and surround

We studied receptive field organization of motion-sensitive neurons in macaque middle temporal cortical area (MT), by mapping direction selectivity in space and in time. Stimuli consisted of pseudorandom sequences of single motion steps presented simultaneously at many different receptive field locations. Spatio-temporal receptive field profiles were constructed by cross-correlating stimuli and spikes. The resulting spike-triggered averages revealed centre-surround organization. The temporal dynamics of the receptive fields were generally biphasic with increased probability for the preferred direction at short latency (50-70 ms) and decreased probability at longer latency (80-100 ms). The response latency of the receptive field surround was on average 16 ms longer than that of the centre. Our results show that surround input and biphasic behaviour reflect two different mechanisms, which make MT cells specifically sensitive to motion contrast in space and time.</p