Interactions, partial interactions, and interaction contrasts in the analysis of variance.

Proposes a 2-step framework for the interpretation of significant 2-treatment interactions. First, a contrast between the means of 1 treatment is estimated separately at each level of the 2nd treatment. A partial interaction test tests the hypothesis that the variability among these contrasts is zero. The 2nd step consists of computing a difference between the separately estimated contrasts. An interaction contrast test tests the hypothesis that this difference is zero. The familywise Type I error rate can be controlled at alpha by employing K. R. Gabriel's (1969) simultaneous test procedure for partial interaction tests and H. Scheffé's (1953) method for interaction contrast tests. (11 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Psychohysical hallucinations of orientation and spatial frequency

After inspection of vertical sinusoidal gratings at least three distinct types of subjective or "hallucinated" patterns can be seen on a uniform test field. One type, here called horizontal streaming (H), is already well-known from the work of MacKay. A second type (V) looks like aroughly sinusoidal grating about 1-5 octaves above the adapting spitial frequency. Under optimal conditions a second vertical component appears at about 2 octaves below the adapting frequency. The third category of aftereffect consists of diagonal lines (D) at two orientations (about +/-40 degrees from vertical). The spatial-frequency band at these two orientations appears to be fairly broad, but roughly similar to the adapting frequency. The duration and strength of D increased, while V declined, at higher adapting spatial frequencies. D and V were increasing functions of adapting contrast, while H appeared abruptly only after the highest adapting contrast. H, D, and V are thus all functionally distinct. A schematic model of cortical organization is proposed to account for these phenomena. Pattern channels selective for a given orientation are grouped together with movement channels selective for the orthogonal direction. Antagonism between channels within such "modules" accounts for the streaming effect (H). Inhibition between modules tuned to different orientations and spatial frequencies accounts for the D and V effects: after adaptation of channels in one module, neighbouring module(s) are released from inhibition to produce a spurious response which is seen as a grating-like object in the adapted part of the visual field. During flickering adaptation a "halluncinated" lattice can be seen superimposed on the adapting grating. It apparently consists of Fourier components more remote from the adapting pattern than D and V are. This disinhibitory effect is strong confirmation of the inhibitory model. The regular and highly organized matrix of channels implied by these experiments may constitute a cortical hypercolumn conducting a coarse, piecewise Fourier transformation of the retinal image.     

Representation of spatial frequency and orientation in the visual cortex

Knowledge of the response of the primary visual cortex to the various spatial frequencies and orientations in the visual scene should help us understand the principles by which the brain recognizes patterns. Current information about the cortical layout of spatial frequency response is still incomplete because of difficulties in recording and interpreting adequate data. Here, we report results from a study of the cat primary visual cortex in which we employed a new image-analysis method that allows improved separation of signal from noise and that we used to examine the neurooptical response of the primary visual cortex to drifting sine gratings over a range of orientations and spatial frequencies. We found that (i) the optical responses to all orientations and spatial frequencies were well approximated by weighted sums of only two pairs of basis pictures, one pair for orientation and a different pair for spatial frequency; (ii) the weightings of the two pictures in each pair were approximately in quadrature (1/4 cycle apart); and (iii) our spatial frequency data revealed a cortical map that continuously assigns different optimal spatial frequency responses to different cortical locations over the entire spatial frequency range.     

Apparent contrast and spatial frequency of local texture elements

We measured the apparent contrast and spatial frequency of a parafoveal Gabor signal located at the center of an array of Gabor signals as a function of both element density and the direction of contrast and spatial frequency of the surrounding elements. The target Gabor appeared lower in contrast and higher in spatial frequency when the elements were in close proximity, regardless of the direction of contrast and spatial frequency of the surrounding elements. Overall, the evidence suggests that the appearance of a parafoveal target is strongly affected by its visual context. These findings provide additional support for the existence of spatial interactions among neurons implicated in textural processing.     

Independent processing across spatial frequency in moving broadband patterns

The aim of the experiments was to discover whether the visual system has independent access to motion information at different spatial scales when presented with a broadband stimulus. Subjects were required to discriminate between a pair of two-frame motion sequences, one containing a coherently displacing pattern and the other containing a pattern with high-frequency noise. The stimuli were either narrowband (1 octave) or broadband (6 octaves spanning 0.23-15.0 cycles deg-1) and their power spectra were either flat or followed a 1/f2 function. For the broadband stimuli, noise was introduced cumulatively into increasingly lower frequencies. For the narrowband stimuli, noise was introduced into the same frequency band as the signal. All stimuli could be defined by the lowest noise frequency (nl) they contained. For each stimulus, the largest spatial displacement across the two frames at which the task could be performed was measured (dmax). For the narrowband stimuli, dmax increased as nl was lowered. This was true over the entire frequency range for the 1/f2 stimuli, though the task became impossible for the flat-spectrum stimuli at the lowest frequencies. This is attributed to the very low contrast of these latter stimuli. The dmax values for the broadband stimuli tended to shadow those of the narrowband stimuli with the equivalent values of nl being around 25% lower. The data were modelled by spatiotemporally filtering the stimuli and considering the amount of directional power in the signal and noise sequences. The results suggest that there must be multiple spatial-frequency channels in operation, and that for broadband patterns the visual system has perceptual access to these individual channel outputs, utilising different filters depending on the task requirements.     

The precision of velocity discrimination across spatial frequency

The precision of velocity coding for moving stimuli of different spatial frequencies was assessed by measuring velocity discrimination thresholds for a 1-c/deg grating paired with a grating whose spatial frequency ranged from 0.25 to 4 c/deg and for grating pairs of the same spatial frequency (0.25, 1, and 4 c/deg). The gratings always moved upward, with velocities ranging from 0.5 to 16 deg/sec. Velocity discrimination was as precise for stimuli that varied in spatial frequency by +/- 2 octaves (0.25 vs. 1 c/deg and 4 vs. 1 c/deg) as for stimuli of the same spatial frequency, for specific ranges of velocity that depended on the spatial and, therefore, the temporal frequencies of the stimuli. Compared with a 1-c/deg grating, the perceived velocity of 4-c/deg gratings was about 1.3 times faster and that of 0.25-c/deg gratings was about 1.3 times slower. Although these perceived velocity biases imply variation of velocity-signal processing among spatial frequency channels, the discrimination results indicate that the motion-sensing system can compare signals across different spatial frequency channels to make fine velocity discrimination within appropriate temporal frequency limits.     

Effects of good form and spatial frequency on global precedence

Does the global precedence effect depend on the goodness of the global form and low spatial frequencies? In Experiments 1 and 2, under a variety of attentional and task conditions, a global advantage in response time (RT) occurred in "good," many-element compound patterns but not in "poor," few-element patterns (unless the local elements were too small to be easily recognized). Symmetric interference effects were found in all patterns, however, suggesting that global and local information were encoded in parallel and that the global advantage in RT involved some postperceptual processes. Experiments 3A and 3B showed that the global advantage in RT and perceived pattern goodness depend on low spatial frequencies: Lowpass-filtered patterns rated as "good" showed the usual global advantage in RT, but highpass-filtered, many-element forms rated as "poor" did not. These findings suggest that a global advantage in RT requires an unambiguous global form conveyed by low spatial frequencies.     

Amygdala size and hypothalamus size predict social play frequency in nonhuman primates: A comparative analysis using independent contrasts.

The amygdala and hypothalamus become sexually differentiated by gonadal hormones giving rise to sexually differentiated behaviors, which include play behavior. Phylogenetic comparative analyses test for relationships between social play and brain structure volumes. Relative volume of the amygdala and hypothalamus correlated with social play, but not nonsocial play, even after controlling for the size of other brain structures. The authors propose that behaviors such as social assessment, recognizing and responding to facial expression, and social response appropriateness, which are mediated by the amygdala, are developed through social play. Additionally, the hypothalamus may regulate the motivation to engage in play through positive reinforcement of pleasurable activity. Thus, the instinctive socioemotive aspects of play in primates appear to be those regulated by the amygdala and hypothalamus. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Binocular matching of dissimilar features in phantom stereopsis

Previously we have demonstrated that quantitative depth perception can be elicited from a stereogram that lacks contrast defined binocular corresponding elements (phantom stereopsis). In this report, we use computer simulation to demonstrate that it is biologically plausible for some known binocular cortical cell types to combine non-conventional matching features. Therefore, binocular matching processes based on the responses of these cells could be a conventional one, namely, looking for similar response patterns in the two eyes. While at cell types we simulated gave identical disparity outputs to the conventional stereogram, they responded differently to the phantom stereogram. Processes other than low-level disparity detectors may have to be invoked in order to achieve a unique depth solution.     

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.     

The effects of eccentricity and spatial frequency on the orientation discrimination asymmetry

This study investigated the effects of eccentricity and spatial frequency on the discrimination of vertical and oblique (10 deg from vertical) Gabor patches. Within a display stimuli were scaled by a factor F = 1 + E/E2 at each eccentricity (E) in an attempt to equate either the number of photoreceptors (E2 = 2.5) or cortical area (E2 = 0.77) engaged at each eccentricity. The task was to detect a differently oriented target among eleven distractors. Orientation discrimination asymmetries (ODAs) were found such that an oblique stimulus was easier to detect in a background of vertical stimuli than vice versa. Subjects were equally sensitive to the two highest frequency Gabor patches and less sensitive to the lowest frequency Gabors. When stimuli were scaled with E2 = 2.5 sensitivity was constant at all eccentricities and the ODA magnitude was unaffected. When stimuli were magnified with E2 = 0.77 both sensitivity and ODA magnitude increased with eccentricity. Generally, we may conclude that the ODA effect is not a strictly foveal phenomenon nor is it a strictly high frequency effect.     

Representation of orientation and spatial frequency in perception and memory: a choice reaction-time analysis

Discrimination and short-term memory for the orientation of sinusoidal gratings that differed in spatial frequency, and for the spatial frequency of gratings that differed in orientation, were measured in a same-different task with 0-10 s interstimulus intervals (ISI) between test and reference stimuli. Introducing a difference between test and reference stimuli on a second dimension, or increasing ISI, did not impair spatial discrimination in terms of accuracy, but choice reaction times for correct decisions were prolonged by both manipulations. Results suggest that perceptual discrimination is based on representations in which orientation and spatial frequency are conjointly coded and that decisions are reached by a serial process scanning multiple-tuned, labeled channels; short-term memory may involve reactivation of these channels.     

The effects of spatial frequency overlap on face recognition.

The effects of spatial frequency overlap between pairs of low-pass versus high-pass images on face recognition and matching were examined in 6 experiments. Overlap was defined as the range of spatial frequencies shared by a pair of filtered images. This factor was manipulated by processing image pairs with high-pass/low-pass filter pairs whose 50% cutoff points varied in their separation from one another. The effects of the center frequency of filter pairs were also investigated. In general, performance improved with greater overlap and higher center frequency. In control conditions, the image pairs were processed with identical filters and thus had complete overlap. Even severely filtered low-pass or high-pass images in these conditions produced superior performance. These results suggest that face recognition is more strongly affected by spatial frequency overlap than by the frequency content of the images. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contrast sensitivity for stationary and drifting spatial frequency gratings in positive- and negative-symptom schizophrenia.

To investigate the transient channel deficit hypothesis in schizophrenia, the authors measured the contrast sensitivity for stationary and drifting gratings in groups with positive- and negative-symptom schizophrenia. The negative-symptom group showed a significant reduction in contrast sensitivity for stationary and drifting gratings at low, medium, and high spatial frequencies in comparison to the control group. There were no differences in contrast sensitivity between the control and positive-symptom groups for stationary and drifting gratings at low-spatial frequencies, but there were significant reductions at medium and high spatial frequencies. The results are consistent with a disorder in both sustained and transient channels in negative-symptom schizophrenia and a disorder in sustained channels in positive-symptom schizophrenia. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of spatial frequency, duration, and contrast on discriminating motion directions

The minimum speed required for discriminating the direction of drifting gratings was measured at a variety of spatial frequencies, display durations, and contrasts. As was reported previously, speed thresholds were relatively constant for middle and high spatial frequencies, but speed threshold was found to be almost inversely proportional to spatial frequency in the range of 0.25 to 1.0 c/deg. Speed threshold was also found to be inversely proportional to duration between 73 and 40 ms. These results at low frequencies and short durations are shown to be consistent with limits set by the spread of energy in the stimuli, producing velocity uncertainty. A quantitative model of temporal filtering is presented that largely accounts for results at all spatial frequencies and durations by the inclusion of constant positional noise. A discussion includes the possible roles of magnocellular and parvocellular mechanisms in mediating speed thresholds.     

Postfusional latency in stereoscopic slant perception and the primitives of stereopsis.

Random dot stereograms of slanted surfaces were constructed, each representing one or two slanted surfaces in different relative arrangements and with different axes. Latency to fusion and from fusion to stereoscopic resolution was measured for each stimulus. It was found that latency to fusion was always very brief but that latency to stereoscopic resolution varied markedly, depending upon the orientation and arrangement of the stereoscopic surfaces. A gradient of discontinuities at a surface boundary produced an instant slant response for that surface, whereas a gradient of absolute disparities across the surface did not, except under conditions where vertical declination (a form of orientation disparity) was present. We conclude that stereopsis is not based on the primitives used in matching the images for fusion and that it is, at least initially, a response to disparity discontinuities which play no role in the fusion process. We also conclude that vertical declination is responded to globally as a slant around a horizontal axis but that other forms of orientation disparity are ineffective. The evidence from our experiments does not support the existence of a stereoscopic ability to respond globally to differences in magnification (or spatial frequency). It is suggested that stereoscopic perception of slant around a vertical axis is slow because it results from the integration of local processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A theoretical analysis of illusory contour formation in stereopsis.

A theoretical analysis of how illusory contours are formed in untextured stereograms is presented that focuses on the role of contour junctions in initiating surface interpolation processes. This theory asserts that the matching geometry of stereoscopic junctions-–that is, the pattern of matchable and unmatchable features present at the intersection of binocularly viewed contours--initiates processes of illusory contour formation (modal completion) and the connection of partially occluded objects (amodal completion). The matching geometry of 3 forms of stereoscopic junctions that elicit percepts of illusory surfaces is derived under the assumption that only horizontal disparities are matched. This analysis reveals the presence of 2 distinct kinds of monocular features generated by binocular viewing. Experiments and demonstrations are presented that reveal the role these monocular features play in modal and amodal completion in stereopsis. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Developmental changes in perception of nonnative vowel contrasts.

Discrimination of 2 German vowel contrasts was examined in English-learning infants of 6–8 and 10–12 mo of age using a head turn procedure. The younger infants were better able than the older infants to discriminate the nonnative contrasts, but performance at 6–8 mo was below levels that have been reported for nonnative consonant contrasts. A 2nd experiment using a habituation looking procedure showed that 4-mo-old infants discriminated both German vowel contrasts, but the 6-mo-olds could not. The findings are consistent with previous consonant work, revealing a shift from a language-general toward a language-specific pattern during the 1st yr of life. However, that shift begins earlier in development for vowels than for consonants. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prolonged muscular flaccidity: frequency and association with unilateral spatial neglect after stroke

A 21-month-old infant developed coma with hypotonia during a viral infection. Acyl CoA dehydrogenase deficiency was diagnosed on the basis of results of the chromatographic study of organic acids performed on a urine specimen collected during the acute episode. However, other disorders of mitochondrial and fatty acid oxygenation can generate similar symptoms. Emphasis is put on the need for collecting urine specimens in patients who develop alterations in consciousness and hypoglycemia without ketonuria during prolonged fasting or repeated vomiting due to a viral infection. Urine chromatography can suggest which enzyme is defective, although the diagnosis should always be confirmed by a study of fatty acid oxygenation in lymphocytes or fibroblasts.