Difference in perceptual and oculomotor responses revealed by apparent motion stimuli presented with an interstimulus interval

Macaque monkeys show reversed ocular following responses to two-frame-motion stimulus presented with inter-stimulus intervals

When two-frame apparent motion stimuli are presented with an appropriate inter-stimulus interval (ISI), motion is perceived in the direction opposite to the actual image shift. Herein, we measured a simple eye movement, ocular following responses (OFRs), in macaque monkeys to examine the ISI reversal effect on oculomotor. Two-frame movies with an ISI induced reversed OFRs. Without ISI, the OFRs to the two-frame movie were induced in the direction of the stimulus shift. However, with ISIs ≥10 ms, OFRs in the direction opposite to the phase shift were observed. This directional reversal persisted for ISIs up to 160 ms; for longer ISIs virtually no ocular response was observed. Furthermore, longer exposure to the initial image (Motion onset delay: MOD) reduced OFRs. We show that these dependences on ISIs/MODs can be explained by the motion energy model. Furthermore, we examined the dependence on ISI reversal using various spatial frequencies. To account for our findings, the optimal frequency of the temporal filters of the energy model must decrease between 0.5 and 1 cycles/°, suggesting that there are at least two channels with different temporal characteristics. These results are consistent with those from humans, suggesting that the temporal filters embedded in human and macaque visual systems are similar. Thus, the macaque monkey is a good animal model for the early visual processing of humans to understand the neural substrates underlying the visual motion detectors that elicit OFRs.

Retinal ON and OFF pathways contribute to initial optokinetic responses with different temporal characteristics

Visual information in the retina is processed via two pathways: ON and OFF pathways that originate from ON and OFF bipolar cells. The differences in the receptors that mediate signal transmission from photoreceptors imply that the response speed to light signals differs between ON and OFF pathways. We studied the initial optokinetic responses (OKRs) of mice using two-frame motion stimuli presented with interstimulus intervals (ISIs) to understand functional difference of these pathways. When two successive image frames were presented with an ISI, observers often perceived motion in the opposite direction of the actual shift. This directional reversal results from the biphasic nature of the temporal filters in visual systems whose characteristics can be estimated from the dependence on ISIs. We examined the dependence on ISIs in the OKRs of TRPM1^-/- mice, whose ON bipolar cells are dysfunctional, as well as in those of wild-type control mice. Wild type and TRPM1^-/- mice showed comparable OKRs in the veridical direction when no ISI was present. Both types of mice showed OKRs that decreased and eventually reversed as the ISI increased, but with a directional reversal at a shorter ISI in TRPM1^-/- than wild-type mice. In addition, the temporal filters of TRPM1^-/- mice estimated from dependence on ISIs were tuned for higher frequencies, suggesting that compared with wild-type mice, the visual system of TRPM1^-/- mice responds to light signals with faster dynamics. We conclude that the ON and OFF pathways contribute to initial OKRs by providing visual signals processed with different temporal resolutions.

Two-frame apparent motion presented with an inter-stimulus interval reverses optokinetic responses in mice

Two successive image frames presented with a blank inter-stimulus interval (ISI) induce reversals of perceived motion in humans. This illusory effect is a manifestation of the temporal properties of image filters embedded in the visual processing pathway. In the present study, ISI experiments were performed to identify the temporal characteristics of vision underlying optokinetic responses (OKRs) in mice. These responses are thought to be mediated by subcortical visual processing. OKRs of C57BL/6 J mice, induced by a 1/4-wavelength shift of a square-wave grating presented with and without an ISI were recorded. When a 1/4-wavelength shift was presented without, or with shorter ISIs (≤106.7 ms), OKRs were induced in the direction of the shift, with progressively decreasing amplitude as the ISI increased. However, when ISIs were 213.3 ms or longer, OKR direction reversed. Similar dependence on ISIs was also obtained using a sinusoidal grating. We subsequently quantitatively estimated temporal filters based on the ISI effects. We found that filters with biphasic impulse response functions could reproduce the ISI and temporal frequency dependence of the mouse OKR. Comparison with human psychophysics and behaviors suggests that mouse vision has more sluggish response dynamics to light signals than that of humans.

Contribution of color signals to ocular following responses

Ocular following responses (OFRs) are elicited at ultra-short latencies (< 60 ms) by sudden movements of the visual scene. In this study, we investigated the roles of color signals in OFRs in monkeys. To make physiologically isoluminant sinusoidal color gratings, we estimated the physiologically isoluminant points using OFRs and found that the physiologically isoluminant points were nearly independent of the spatiotemporal frequency of the gratings. We recorded OFRs induced by the motion of physiologically isoluminant color gratings and found that OFRs elicited by the motion of color gratings had different spatiotemporal frequency tuning from those elicited by the motion of luminance gratings. Additionally, OFRs to isoluminant color gratings had smaller peak responses, suggesting that color signals weakly contribute to OFRs compared with luminance signals. OFRs to the motion of stimuli composed of luminance and color signals were also examined. We found that color signals largely contributed to OFRs under low luminance signals regardless of whether color signals moved in the same or opposite direction to luminance signals. These results provide evidence of the multichannel visual computations underlying motor responses. We conclude that, in everyday situations, color information contributes cooperatively with luminance information to the generation of ocular tracking behaviors.