Lynn A. Olzak

Neural recoding in human pattern vision: model and mechanisms.

We describe a model of neural recoding in spatial vision that specifies how the outputs of selected units akin to VI cells are normalized and combined to signal information about particular stimulus attributes. The recoding portion of the model is linked to psychophysical behavior via a two-stage signal-detection decision module that specifies how the outputs of the combining mechanisms are used in making fine spatial discriminations. We describe how masking and cue summation experiments isolate each of the processing stages, how earlier results from such studies guided development of the model, and we demonstrate how these procedures permit empirical estimates of model parameters as well as tests of alternative formulations. An important part of our work describes the characteristics of two complementary types of higher-level mechanisms isolated from previously published discrimination data. One sums normalized primary-level responses over disparate frequencies to signal precise information about the orientation of a stimulus; the other sums over all orientations to signal the spatial grain of texture-like patterns. We demonstrate how the model accounts for a large body of previously published discrimination data, and present the results of a new quantitative test of model predictions.

When orthogonal orientations are not processed independently

We studied the extent to which two gratings, superimposed at orthogonal orientations, are processed independently in making spatial frequency discriminations. Our findings suggest that independence is not preserved in this task, and that it breaks down in two distinct ways: (1) judgments about one component are reduced in accuracy by the presence of the second; and (2) information about the two component gratings is not used independently in making judgments about the compound stimulus formed by them. These results place constraints on the applicability of models that assume information from spatially tuned mechanisms is directly available in performing a discrimination task.

Configural effects constrain fourier models of pattern discrimination

Many models of spatial pattern discrimination assume that judgments are based on information directly available from mechanisms tuned to limited ranges of spatial frequency and orientation. We tested the validity of this assumption for spatial frequency, orientation, and contrast information in a series of complex pattern discrimination experiments. Observers discriminated between simple gratings, between gratings masked by components that differed widely in spatial frequency and/or orientation, and between patterns that presented two cues to discrimination, one in each frequency and/or orientation band. Component cues were combined either in rigid-object correspondence (e.g. both components were rotated clockwise in one pattern and counterclockwise in the other) or in opposition (e.g. in one pattern one component was rotated clockwise, the other counterclockwise; the direction of rotation was reversed for each component in the second pattern). The results demonstrate that information from tuned pathways is not always used directly in making spatial judgments, but in some cases is combined across wide regions of the Fourier domain prior to the discrimination decision. We find two distinct patterns of combination that appear to independently signal information about texture and edges. These findings provide a potential link between low-level, spatially tuned analyzers in the visual system and higher-level pattern processing mechanisms.

Multiple gain control processes in contrast–contrast phenomena

Spatial interactions among orientation-tuned gain control processes are presumed to mediate center-surround contrast-contrast phenomena. In this paper, we assess contributions of gain control processes that pool over orientation. We measured the apparent contrast of a luminance-modulated center disk embedded in various modulated surrounds. In all conditions, observers compared the apparent contrast of the test center to an identically modulated disk with no surround. When center and surround are simple, vertical sinusoids and presented in phase, suppression depends upon surround contrast and is marked at high contrasts. When components are presented 180 degrees out of phase, no suppression occurs at any contrast. When a horizontal component is added to the surround, much less suppression occurs. However, strong suppression is reinstated when both center and surround are plaids. Neither of the latter two effects are phase dependent. We suggest that two different sources of gain control are revealed by the simple sinusoidal and the plaid stimuli. One is orientation tuned and phase-dependent. The other pools over all orientations and includes neurons tuned to multiple phases.

It's not easy being green: student recall of plant and animal images

It is well documented that people are less interested in studying plants than animals. We tested whether university students would selectively recall more animal images than plant images even when equally-nameable plant and animal images were presented for equal lengths of time. Animal and plant images were pre-tested and 14 animal-plant pairs were selected, based on student ability to equally name the images. These images were randomly presented to two groups of university students: those currently enrolled in a psychology class and those currently enrolled in a botany class. Student recall of each image was recorded after a distracting task. The results confirmed that the animal images were recalled significantly more than the plant images. There was no apparent effect of attending a botany class on these results. However, gender effects were identified for recall of plant versus animal images in general (women recalled more plants than men) and for four specific plant images (carnation, rose, daisy, and venus fly trap). When teaching biology, teachers should present equal numbers of plant and animal examples and use the most memorable plant images possible to attempt to offset student selective attention to animals.

Discrimination of Complex Patterns: Orientation Information is Integrated across Spatial Scale; Spatial-Frequency and Contrast Information are Not

Real-world objects are complex, containing information at multiple orientations and spatial scales. It is well established that at initial cortical stages of processing, local information about an image is separately represented at multiple spatial scales. However, it is not yet established how these early representations are later integrated across scale to signal useful information about complex stimulus features, such as edges and textures. In the studies reported here, we investigate the scale-integration processes involved in distinguishing among complex patterns. We use a concurrent-response paradigm in which observers simultaneously judge two components of compound gratings that differ widely in spatial frequency. In different experiments, each component takes one of two slightly different values along the dimensions of spatial frequency, contrast, or orientation. Using analyses developed within the framework of a multivariate extension of signal-detection theory, we ask how information about the frequency, contrast, or orientation of the components is or is not integrated across the two grating components. Our techniques permit us to isolate and identify interactions due to excitatory or inhibitory processes from effects due to noise, and to separately assess any attentional limitations that might occur in processing. Results indicate that orientation information is fully integrated across spatial scales within a limited orientation band and that decisions are based entirely on the summed information. Information about spatial frequency and contrast is not summed over spatial scale; cross-scale results show sensory independence. However, our results suggest that observers cannot simultaneously use information about frequency or contrast when it is presented at different spatial scales. Our results provide direct evidence for the existence of a higher-level summing circuit tailored to signal information about orientation. The properties of this mechanism differ substantially from edge-detector mechanisms proposed by Marr and others.

Gratings: why frequency discrimination is sometimes better than detection

Two models that assume independent processing among frequency-selective analyzers are presented. These are a distance model and an integration model derived from signal-detection theory. The models permit quantitative comparisons between detection and discrimination performance and lead to an empirical comparison that is sensitive to effects of correlated noise and interactions among the responding mechanisms. The stimuli were four pairs of sine-wave gratings that differed in spatial-frequency separation. They were presented to observers in a signal-detection rating paradigm, which was used for both detection and discrimination judgments. The results indicate the presence of properties, such as inhibitory interactions or correlated noise among responding mechanisms, that favor discrimination over detection.

Contrast gain control and fine spatial discriminations

The effect of contrast gain control mechanisms on discrimination between highly similar simple and complex stimuli is examined, with a focus on how discrimination accuracy changes as a function of the contrast of stimulus components. Two models of contrast gain control are evaluated. In both, the response of each pathway is attenuated by a factor determined by the total activity in a large pool of pathways. One model bases attenuation on the sum of linear filter responses within this pool; the other, based on Heegers contrast energy-driven model [J. Neurophysiol. 70, 1985 (1993)], uses squared filter response. Predictions generated from the models are compared with data from experiments reported here and from the literature. Predictions are made for simple grafting stimuli of different sizes and for stimuli to which a second grafting component is added either as a second cue or as a mask. With one exception, predictions of the models agree closely with each other and with the data. The exception is a masking study that differentiates the models and supports the filter-driven model over the energy-driven model.

Uncertainty experiments support the roles of second-order mechanisms in spatial frequency and orientation discriminations

Previous studies of spatial frequency and orientation discrimination [Vision Res. 32, 1885 (1992)] suggest the existence of two second-order cortical mechanisms: one that mediates spatial frequency discriminations and sums signals across orientations and one that mediates orientation discriminations and sums signals across spatial frequency bands. The existence of each mechanism is tested in an uncertainty experiment in which the observer does not know which of two hypothetically pooled signals deviates from the standard but must judge whether the deviation is an increment or a decrement. No uncertainty effect is expected if the signals are completely pooled. Observed effects are compared with this expectation and with both theoretical and empirical estimates of the effects expected if the signals are processed separately. Results support the existence of the first mechanism, but not its exclusive role in mediating spatial frequency judgments, and support the exclusive role of the second mechanism in mediating orientation judgments.

Cue summation in spatial discriminations

We investigated the extent to which discrimination performance improves when more than a single cue distinguishes two patterns. When two simple gratings differ slightly in spatial frequency, orientation, and/or contrast, the performance of most observers is better in multiple-cue conditions than in single-cue conditions and by an amount indicating Euclidean summation of information. These results are shown to be consistent with discrimination models that integrate information over different, spatially selective pathways. However, little summation is found when the task requires integration of cues across widely separated spatial frequency bands. This result implies that information is integrated only over pathways tuned to a common region of the spatial frequency domain.