On the response of simple and complex cells to random dot patterns: a reply to skottun, grosof and de valois

The spatio-temporal requirements for direction selectivity were studied in two extrastriate motion processing areas in the cat, area 18 and the posteromedial lateral suprasylvian cortex (PMLS). Direction, velocity and pixel size of random pixel arrays (RPA) were adjusted for each neuron and direction selectivity was measured as a function of step size and delay for a given optimal velocity. A subset of direction selective complex cells in area 18 was tuned to intermediate step size and delay combinations rather than the smoothest motion (band-pass cells). Other area 18 complex cells responded best to the smallest value of step size and delay (low-pass cells). Tuning varied with the pixel size of the RPA. Cells with tuning for smaller pixels favoured a preference for non-smooth motion. Area 18 cells with lower spatial resolution showed larger optimal and maximal step sizes. For a subset of the cells in area 18, we measured direction selectivity for extensive step-delay combinations, covering multiple velocities. Results showed that most cells were tuned to narrow range of step-delay combinations, and that the optimal step size was independent of temporal delay. Direction selective complex cells in PMLS were tuned to larger pixel sizes than those in area 18, although the distributions did overlap. In contrast to area 18, PMLS cells preferred the smoothest motion, irrespective of RPA pixel size.

Viewing-distance invariance of visual perception has evolutionary advantages, but it is of necessity limited by spatial and temporal resolution. Even within these resolution limits viewing-distance invariance might not be perfect or even good, but there are remarkably few studies of its precise limits. Here we ask to what extent viewing-distance invariance holds for motion aftereffects (MAEs). There are (at least) two different MAEs: one can be seen on a static test pattern (sMAE) and is tuned to low speeds, the other only becomes manifest on a dynamic noise test stimulus (dMAE) and is sensitive to higher adaptation speeds. We show that each of these MAEs has a limited viewing-distance invariance, the dMAE only for higher screen-speeds and the sMAE only for lower screen-speeds. In both cases upper angular-speed limits shift to higher values for smaller viewing-distances (lower spatial frequencies, larger fields). This upper limit is constant, independent of viewing distance, if expressed in terms of screen-speed. On the other hand the lower speed limit is fixed in angular-speed and variable in screen-speed terms. Explanations for these findings are provided. We show that there is no fixed optimum viewing-distance or optimum angular stimulus-size for either of the two MAEs.

The characteristics of directionally selective cells in area 17 of the cat are studied using moving random pixel arrays (RPAs) with 50% white and 50% black pixels. The apparent motion stimulus is similar to that used in human psychophysics [Fredericksen et al. (1993). Vision Research, 33, pp. 1193–1205]. We compare motion sensitivity measured with single-step pixel lifetimes and unlimited pixel lifetimes. A motion stimulus with a single-step pixel lifetime contains directional motion energy primarily at one combination of spatial displacement and temporal delay. We recorded the responses of complex cells to different combinations of displacement and delay to describe their spatio-temporal correlation characteristics. The response to motion of RPAs with unlimited lifetime is strongest along the preferred speed line in a delay vs displacement size diagram. When using an RPA with a single-step pixel lifetime, the cells are responsive to a much smaller range of spatial displacements and temporal delays of the stimulus. The maximum displacement that still gives a directionally selective response is larger when the preferred speed of the cell is higher. It is on average about three times smaller than the receptive field size. © 1997 Elsevier Science Ltd. All rights reserved.