Responses of visual cells in cat superior colliculus to relative pattern movement

Self-motion trajectories can facilitate orientation-based figure-ground segregation

Segregation of objects from the background is a basic and essential property of the visual system. We studied the neural detection of objects defined by orientation difference from background in barn owls (Tyto alba). We presented wide-field displays of densely packed stripes with a dominant orientation. Visual objects were created by orienting a circular patch differently from the background. In head-fixed conditions, neurons in both tecto- and thalamofugal visual pathways (optic tectum and visual Wulst) were weakly responsive to these objects in their receptive fields. However, notably, in freely viewing conditions, barn owls occasionally perform peculiar side-to-side head motions (peering) when scanning the environment. In the second part of the study we thus recorded the neural response from head-fixed owls while the visual displays replicated the peering conditions; i.e., the displays (objects and backgrounds) were shifted along trajectories that induced a retinal motion identical to sampled peering motions during viewing of a static object. These conditions induced dramatic neural responses to the objects, in the very same neurons that where unresponsive to the objects in static displays. By reverting to circular motions of the display, we show that the pattern of the neural response is mostly shaped by the orientation of the background relative to motion and not the orientation of the object. Thus our findings provide evidence that peering and/or other self-motions can facilitate orientation-based figure-ground segregation through interaction with inhibition from the surround.NEW & NOTEWORTHY Animals frequently move their sensory organs and thereby create motion cues that can enhance object segregation from background. We address a special example of such active sensing, in barn owls. When scanning the environment, barn owls occasionally perform small-amplitude side-to-side head movements called peering. We show that the visual outcome of such peering movements elicit neural detection of objects that are rotated from the dominant orientation of the background scene and which are otherwise mostly undetected. These results suggest a novel role for self-motions in sensing objects that break the regular orientation of elements in the scene.

Distributed dendritic processing facilitates object detection: a computational analysis on the visual system of the fly

Background

Detecting objects is an important task when moving through a natural environment. Flies, for example, may land on salient objects or may avoid collisions with them. The neuronal ensemble of Figure Detection cells (FD-cells) in the visual system of the fly is likely to be involved in controlling these behaviours, as these cells are more sensitive to objects than to extended background structures. Until now the computations in the presynaptic neuronal network of FD-cells and, in particular, the functional significance of the experimentally established distributed dendritic processing of excitatory and inhibitory inputs is not understood.

Methodology/Principal Findings

We use model simulations to analyse the neuronal computations responsible for the preference of FD-cells for small objects. We employed a new modelling approach which allowed us to account for the spatial spread of electrical signals in the dendrites while avoiding detailed compartmental modelling. The models are based on available physiological and anatomical data. Three models were tested each implementing an inhibitory neural circuit, but differing by the spatial arrangement of the inhibitory interaction. Parameter optimisation with an evolutionary algorithm revealed that only distributed dendritic processing satisfies the constraints arising from electrophysiological experiments. In contrast to a direct dendro-dendritic inhibition of the FD-cell (Direct Distributed Inhibition model), an inhibition of its presynaptic retinotopic elements (Indirect Distributed Inhibition model) requires smaller changes in input resistance in the inhibited neurons during visual stimulation.

Conclusions/Significance

Distributed dendritic inhibition of retinotopic elements as implemented in our Indirect Distributed Inhibition model is the most plausible wiring scheme for the neuronal circuit of FD-cells. This microcircuit is computationally similar to lateral inhibition between the retinotopic elements. Hence, distributed inhibition might be an alternative explanation of perceptual phenomena currently explained by lateral inhibition networks.

Dynamic response properties of visual neurons and context-dependent surround effects on receptive fields in the tectum of the salamander Plethodon shermani

Neuronal responses to complex prey-like stimuli and rectangles were investigated in the tectum of the salamander Plethodon shermani using extracellular single-cell recording. Cricket dummies differing in size, contrast or movement pattern or a rectangle were moved singly through the excitatory receptive field of a neuron. Paired presentations were performed, in which a reference stimulus was moved inside and the different cricket dummies or the rectangle outside the excitatory receptive field. Visual object recognition involves much more complex spatial and temporal processing than previously assumed in amphibians. This concerns significant changes in absolute number of spikes, temporal discharge pattern, and receptive field size. At single presentation of stimuli, the number of discharges was significantly changed compared with the reference stimulus, and in the majority of neurons the temporal pattern of discharges was changed in addition. At paired presentation of stimuli, neurons mainly revealed a significant decrease in average spike number and a reduction of excitatory receptive field size to presentation of the reference stimulus inside the excitatory receptive field, when a large-sized cricket stimulus or the rectangle was located outside the excitatory receptive field. This inhibition was significantly greater for the large-sized cricket stimulus than for the rectangle, and indicates the biological relevance of the prey-like stimulus in object selection. The response properties of tectal neurons at single or paired presentation of stimuli indicate that tectal neurons integrate information across a much larger part of visual space than covered by the excitatory receptive field. The spike number of a tectal neuron and the spatio-temporal extent of its excitatory receptive field are not fixed but depend on the context, i.e. the stimulus type and combination. This dynamic processing corresponds with the selection of the stimuli in the visual orienting behavior of Plethodon investigated in a previous study, and we assume that tectal processing is modulated by top down processes as well as feedback circuitries.

The invariance of directional tuning with contrast and coherence

The responses of motion mechanisms depend not only on the direction of a stimulus, but also on its contrast, coherence and speed. We examined how contrast, coherence and directional selectivity interact by measuring directional tuning psychophysically across a wide range of coherence and contrast levels. We fit data with a simple model that estimated directional tuning bandwidth using contrast and coherence gain parameters that were based on neurophysiological estimates. This model estimated a bandwidth of approximately 90 degrees for directionally selective mechanisms. Bandwidth was invariant across a wide range of contrasts and coherences, as predicted by models of contrast normalization.

Cellular response to texture and form defined by motion in area 19 of the cat

The present study examined the neuronal sensitivity in area 19 of the cat to a motion-defined bar and to texture. Sensitivity was tested in normal, lesioned (areas 17-18) and split-chiasm cats using a kinematogram, as well as a textured bar drifting on a uniform light background and a light bar drifting on a stationary textured background. Texture density was varied. The results indicate that almost all cells of area 19 recorded in the three groups of cats responded to a motion-defined bar or to its edges. Texture density influenced the responses in that the discharge rate increased as density decreased. However, the majority of cells were sensitive to the highest texture density kinematogram. Moreover, the neural responses of all cats were either independent of the density of the textured bar or background, or were modulated by it. These results show that cells in area 19 can signal the presence of a kinetic bar and that the density of either the textured bar, the background or both can influence figure-ground detection. The results are interpreted with respect to how various inputs influence the function of area 19.

Coding for stimulus velocity by temporal patterning of spike discharges in visual cells of cat superior colliculus

Statistical analyses, performed on extracellularly recorded spike trains generated by 69 single motion sensitive visual cells in the intermediate layers of superior colliculi of pretrigeminal cat preparations, revealed that--even in the unstimulated condition (38/69)--most neuronal spike discharge patterns tended to switch between two stochastically distinct states, in the form of rapidly alternating "bursting" (high frequency) and "resting" (low frequency) episodes. The numbers of consecutive interspike intervals within a given state were, as a rule, independent integer-valued random variables with discrete probability distributions, in essential agreement with the semi-Markov model proposed by Ekholm and Hyvärinen [(1970) Biophysical Journal, 10, 773-796]. The introduction of visual stimuli (47/69) moving with velocities of 2-160 deg/sec caused systematic and reproducible changes in the ratio of bursting to resting activities, decreases in overall discharge variability, and increases in signal transinformation flow. Moreover, with one group of stimulated cells (28/47), increasing stimulus velocity caused increasingly precise ("stimulus-forced") synchronization of bursting episodes with specific phases of stimulus movement; while for a smaller group (12/47), stimulus-related alternations between bursting and resting states assumed the form of semi-rhythmical burst discharges within the characteristic 60-80 Hz "gamma oscillation" range ("stimulus-induced" synchronization). For a minority of cells (7/47), switching between bursting and resting states--although characteristically modified by stimulus velocity--remained largely desynchronized with all phases of stimulus transit. It was argued that such temporal patterns of discharge may constitute elements of a candidate "distribution" code for movement detection by the cat visual system.

Detecting slant-in-depth of real trapezoidal and rectangular surfaces: moving-monocular viewing equivalent to stationary-binocular viewing

Cues from binocularity and observer motion are often believed to be more important in perceiving depth than pictorial cues such as relative visual size and linear perspective. Both binocularity and motion are effective in simulated displays. However, for real stimuli evincing nonveridical pictorial cues, binocularity has been more effective than motion; sometimes motion has had an insignificant effect. This may reflect inadequate extent of motion, an assertion investigated in the present study. Two groups of observers determined whether rectangular and trapezoidal surfaces were slanted-in-depth under stationary-monocular (SM), stationary-binocular (SB), and moving-monocular conditions with 15-cm (15MM) and 25-cm (25MM) lateral head-motion extents according to group. The trapezoidal surfaces appeared as rectangular during SM viewing to mislead regarding slant. The effect of pictorial cues was substantially diminished during SB viewing whereas 15MM viewing was weak, 25MM was as effective as SB viewing. Comparison of the overall numbers of correct responses for the two groups indicated no contextual biasing.

Sensitivity to relative and absolute motion

The threshold of sensitivity to movement could be governed by mechanisms that are sensitive either to change in spatial position, or directly to the movement itself. The use of spatially complex patterns (random-dot patterns) has been suggested to eliminate the former strategy allowing examination of the movement detecting mechanisms in isolation. By means of such a technique, thresholds for directional judgements were determined for patterns which underwent either a simple displacement or a shearing displacement. Thresholds for shearing motion were found to be around one half of those for simple motion, suggesting that relative, rather than absolute, motion governs performance for small displacements. This contrasts with previous experiments which showed that absolute motion governs performance for much larger displacements.

Primary depth cues and background pattern in the portrayal of slant

A rectangularity postulate has been used in algorithms for the purpose of interpreting two-dimensional representations of rectilinear objects. This rectangularity postulate may affect the perception of true surfaces. In this study, rectangular surfaces and trapezoidal surfaces--the latter simulating the horizontal slant-in-depth of the rectangular surfaces--were viewed under static-monocular, moving-monocular, and static-binocular conditions, both with and without a background pattern. The static-binocular condition elicited the greatest number of accurate responses. The moving-monocular condition did not elicit significantly more accurate responses than the static-monocular viewing condition did. The effect of background pattern was insignificant. These results were unexpected in terms of ecological validity and (regarding moving-monocular viewing) because of the importance of the role of relative visual motion in the detection of object motion. However, the results are consistent with the perception of depth separation of two discrete objects.

After-effects of moving textured background in motion-sensitive neurons of anuran optic tectum

Motion-sensitive neurons in anuran optic tectum were shown to respond to a stationary object centered in the excitatory receptive field, if a textured background moved for a while and then stopped ('motion after-response'). This motion after-response was attributed to a post-inhibitory rebound activation derived from effects of masking the excitatory receptive field center surrounded by an antagonistic inhibitory region. It was suggested that a similar rebound activation mechanism may also be involved in a certain type of perceptual motion after-effects in humans.

Deep tectal cells in pigeons respond to kinematograms

Deep tectal neurons in pigeons respond selectively to moving visual stimuli, and are inhibited by large background patterns moved in-phase with these stimuli. In this investigation we demonstrate that these same deep tectal neurons respond equally well to kinematograms as they do to traditional luminance contrast stimuli typically employed in visual experiments. Computer generated kinematograms, the motion domain equivalents of random dot stereograms, were used as stimuli in these experiments. These kinematograms, where a small centrally located set of random dots is moved coherently in one direction while the remaining dots are moved in a different direction, thus constitute a pure motion stimulus where the stimulus form is only visible in the dynamic pattern, but does not exist on any single frame. Both 'object' configured and 'hole' configured kinematograms were employed; the former appearing as regions of texture moving over, or in front of, the background texture, while the latter appear as windows through which a more distant textured surface is revealed. Extracellular recordings from isolated deep tectal cells showed that all units responded in a very similar manner whether the stimulus was an 'object' configured kinematogram or the more traditional luminance contrast variety. This similarity included directional selectivity, the in-phase inhibition anti-phase facilitation effect, and sensitivity to opposed motion independent of direction. However, when the kinematograms were configured as 'holes' none of the units tested responded to these stimuli. The significance of these observations for tectal functioning, image segmentation through motion and animal camouflage is discussed.

Aftereffect of induced rotation: number of pattern elements and area of pattern in inducing stimulus

Previous research has suggested that aftereffect of induced rotation can be longer when the number of pattern elements in the inducing stimulus is greater. This was explained by the amount of shearing between pattern elements of inducing and static stimuli. However, the results might also be explained by the greater area occupied by a greater number of pattern elements. The present experiment used three inducing stimuli, in which number of pattern elements and area of pattern were varied independently, to test between these hypotheses. The former was corroborated and the latter refuted.

Visual receptive field properties in the posterior suprasylvian cortex of the cat: a comparison between the areas PMLS and PLLS

Receptive field (RF) properties of single units were examined in two visual areas in the cat's postero-medial and postero-lateral suprasylvian cortex (PMLS/PLLS). Electrophysiological recordings were made in the paralyzed and anesthetized preparation in corresponding regions of the medial and lateral banks of the lateral suprasylvian sulcus (LS). In both areas, cell samples were obtained from within the same range of A/P co-ordinates. In both samples, cells responded best to moving stimuli, had large RF's, and did not differ with respect to the distributions of their ocular dominance. Binocularity was equally high, but units in PLLS showed significantly less binocular summation. Cells in both areas preferred high velocities, and a high percentage was direction selective, with directions up and away from the vertical meridian being most common. While in PMLS the preferred direction of a unit usually could be classified into a "radial" vs a "circular" category when RF position was taken into account, this was not possible in PLLS. Units in PLLS also had significantly higher spontaneous activity, higher optimal velocity, and larger RF sizes. In PLLS the investigation of background-foreground interactions revealed a large variety of phase-dependent responses. This is in contrast to the clear preponderance of in-phase inhibition and antiphase facilitation effects in PMLS. The results indicate important differences of RF properties for the two areas, but do not yet suggest a clear functional differentiation. These differences reflect in some respects the RF properties of cells in the structures that provide the main afferents to the two areas.