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PubMed2003

Representation of statistical properties.

Everyday scenes often contain sets of similar objects. Perceptual representations may summarize these with statistical descriptors. After determining the psychological mean of two sizes, we measured thresholds for judging the mean with arrays of 12 circles of heterogeneous sizes. They were close to those for the size of elements in homogeneous arrays and single elements, and were little affected by either exposure duration (50-1000 ms) or memory delays (up to 2s). They were only slightly more accurate within the same distribution than across different distributions (normal, uniform, two-peaks, and homogeneous), confirming that subjects were indeed averaging sizes.

Discrimination, Psychological↗
PubMed1998

Feature asymmetries in visual search: effects of display duration, target eccentricity, orientation and spatial frequency.

In Experiments 1-3, we monitored search performance as a function of target eccentricity under display durations that either allowed or precluded eye movements. The display was present either until observers responded, for 104 msec, or for 62 msec. In all three experiments an orientation asymmetry emerged: observers detected a tilted target among vertical distracters more efficiently than a vertical target among vertical distracters. As target eccentricity increased, reaction times and errors augmented, and the set size effect became more pronounced, more so for vertical than tilted targets. In Experiments 4-7, the stimulus spatial properties were manipulated: spatial frequency; size; and orientation. The eccentricity effect was more pronounced for vertical than tilted targets and for high- than low-spatial frequency targets. This effect was eliminated when either the size, the size and orientation, or the size and spatial frequency were magnified (M-cortical factor). By increasing the signal-to-noise ratio, magnification reduced the extent of both asymmetries; it aided more the detection of tilted than vertical and of high- than low-spatial frequency targets. Experiments 4-7 indicate that performance improvement in the magnified conditions was due to the specific pairing of stimulus size with retinal eccentricity and not to the larger stimulus size of the magnified conditions. We conclude that stimulus size, orientation and spatial frequency influence the extent of the eccentricity effect and the efficiency of search performance.

Eye Movements↗
PubMed1999

Context-dependent changes in visual sensitivity induced by Müller-Lyer stimuli.

We measured the detectability of a single line (target) flanked by high-contrast inward- or outward-pointing arrowheads (context). We show that as a function of target contrast, context angle, and context position there is a continuum of contextual modulations of target detectability that vary from strong inhibition (target detection is impaired) to strong excitation (target detection is facilitated), but target detection is not affected when the context is presented at low contrasts. The results show striking correlations with the perceived length distortions in the Müller-Lyer illusion, i.e. an inward-pointing arrowhead results in improved target detectability and increased perceived length of the bar, whereas an outward-pointing arrowhead results in diminished target detectability and decreased perceived length of the bar. Both suppressive and facilitatory effects diminish as target contrast, arrowhead angle, and line-arrowhead spatial disparity are increased. At larger distances between line and arrowhead the suppressive effects become facilitatory (the Müller-Lyer illusion reverses). When concurrent Müller-Lyer extent experiments are run, we found that the perceived length of the target stimulus is overestimated or underestimated as it is flanked by high-contrast inward or outward-pointing arrowheads, the magnitude of the length distortion effects diminishing as target contrast increases. To explain the nature of both context-induced suppression and facilitation in contrast detection we present a population model of orientation detectors in visual cortex that relies on short and long-range horizontal cortical connections, and suggest that that the same type of mechanism that accounts for contrast detection may account for perceived extent.

Adult↗
PubMed1999

Contrast integration across space.

Contrast integration across space was studied in respect to stimulus extent and the spatial layout, using high-contrast stimuli. Contrast discrimination thresholds were measured (2AFC) by either increasing the size of a peripheral (2.4 degrees) Gabor signal (GS: lambda = 0.08 degree) or by increasing the number of GS elements in a circular arrangement. The supra-threshold mask (pedestal) was either increased with the target or fixed at maximal size and had 30% contrast. For stimuli with an increasing size of both the pedestal and the increment target, we find approximately constant discrimination thresholds. Contrast discrimination improved linearly on a log-log scale with slopes average of -1/4 (fourth-root summation) when the size of the Gabor target was increased but the mask was kept at maximal size, indicating contrast integration across space. Taken together, these results indicate balanced spatial integration of both contrast increment and pedestal, resulting object-size invariant contrast discrimination. Contrast discrimination was found to improve as well when the number of aligned Gabor elements was increased (both pedestal and increment), pointing to independent contrast normalization for disconnected (sparsely positioned) stimuli. The results indicate a complex pattern of spatial integration involved in contrast discrimination, possibly depending on image segmentation.

Contrast Sensitivity↗
PubMed2000

A size illusion of the letter 'P'.

The perceived size of 'P' and 'p' is influenced by their vertical position relative to nearby letters. In the experiments reported here, we show that uppercase 'P' appears smallest when it is displaced downward toward the letter's lowercase position, whereas lowercase 'p' appears largest when it is near the uppercase position. Our results show that this P-illusion not only occurs in the presence of nearby letters, but is also found when 'P' is displaced relative to a nearby horizontal line. This type of size illusion seems to clearly occur only with 'P' and 'p', and we suggest that it is a result of learning to read and recognize letters in the English alphabet.

Humans↗
PubMed1999

Dynamics of molecules involved in antigen presentation: effects of fixation.

Antigen presentation by MHC class II molecules can be enhanced by paraformaldehyde fixation of antigen-presenting cells prior to assay. This treatment might be expected to aggregate membrane proteins and thus stabilize and strengthen transient protein-protein interactions involved in intercellular cooperation. Lateral and rotational dynamics of the MHC class II antigen I-Ad on A20 cells fixed with various concentrations of paraformaldehyde were examined by fluorescence photobleaching recovery and time-resolved phosphorescence anisotropy, respectively. Probes were tetramethylrhodamine and erythrosin conjugates of MKD6 Fab fragments. Increasing concentrations of paraformaldehyde led to a progressive increase in the limiting anisotropy of I-Ad at 4 degrees C from the value of 0.042 for untreated cells, indicative of large aggregate formation, while leaving the rotational correlation time of 29 micros unchanged, a measure of the unperturbed molecule. On the other hand, the translational diffusion constants decreased from approximately 2x10(-10) cm2 s(-1), while the fractional recovery remained unchanged at about 40-50%. Taken together, these results suggest that fixation crosslinks class II molecules to each other or to other membrane proteins into structures large enough (>500,000 kDa) to diffuse translationally with perceptibly size-dependent rates. The fixation effects on both class II rotation and lateral diffusion were half-maximal at paraformaldehyde concentrations of approximately 0.2%. Possible relations between the biological effector functions of class II and the physical sizes of fixation-induced aggregates are discussed.

Animals↗
PubMed1990

The effect of stimulus size on the measured threshold values in automated perimetry.

Patients with severely depressed visual fields as determined by automated static perimetry are often tested with larger test stimuli in order to obtain meaningful data. "Normal" threshold values are known and included in some perimeters' data analysis programs for stimulus III, but not for larger stimuli. Using a user-defined program on the Octopus 201 perimeter to determine mean retinal sensitivity, a study was conducted in 17 normal subjects to determine the effect of stimulus size on measured threshold. With stimulus III, the mean retinal sensitivity within the central 30 degrees was 28.4 +/- 2.3 dB; it was 31.9 +/- 2.3 dB for stimulus IV, and measured 36.00 +/- 2.5 dB with stimulus V. The differences between the values are statistically significant (P less than 0.001). It is not known whether these differences can be generalized from normals to patients with glaucoma or other diseases.

Adult↗
PubMed2000

Skilled reaching an action pattern: stability in rat (Rattus norvegicus) grasping movements as a function of changing food pellet size.

Many animals in different mammalian orders display skilled forelimb use for prehension, but there has been little study of the rules underlying skilled forelimb use that may provide insight into its neural organization. Inflexibility of movement would imply an underlying fixed neural control characteristic of innate action pattern, whereas flexibility of movement would imply more plastic neural control. In the present study, rat reaching was examined by analyzing movements used to obtain nine different sized food pellets, weighing between 20 and 1000 mg. The prediction was that if the rats' reaching movements were flexible, then systematic modifications in reaching would occur as food pellet size changed; whereas if reaching was inflexible, disruptions should occur for extreme food pellet sizes. Reaching was filmed using normal (30 frames/s) and high-speed (60 frames/s) video-recording procedures. Behavior was scored in terms of successful reaches and, in addition, a new rating scale, derived from Eshkol-Wachman Movement Notation, was used to evaluate the qualitative aspects of movement. Reaching success was stable across smaller food pellet sizes but dropped sharply for larger food pellets sizes. Qualitative analysis of limb movements revealed that animals were unable to change their typical movement sequence to obtain the larger pellets. This result indicates that rat skilled reaching is relatively inflexible, supporting the position that it is produced by a complex, relatively fixed neural circuitry. This finding is discussed in relation to the evolution of skilled reaching, the modifications of skilled reaching that are observed after motor cortex and other nervous system injuries, and comparative differences in reaching in rats and primates.

Animals↗
PubMed2002

Independent control of the digits predicts an apparent hierarchy of visuomotor channels in grasping.

If an object changes position at the onset of a reach-to-grasp movement, both the transport speed and the grip aperture are adjusted. If the object changes in size at the onset, only the grip aperture is adjusted. This combination of results has been interpreted as being the consequence of a hierarchical relationship between visuomotor channels for transport and grip. We argue that our alternative view on grasping can account for the observed behaviour without making new assumptions. In our view, grasping consists of smooth (minimal jerk) movements of each digit to a target position on the object. The digits' target positions change, both when object position and when object size change. A model in which the individual digits move smoothly to these new positions yields the same behaviour as is observed experimentally.

Algorithms↗
PubMed2004

Passive, active and intra-active (self) touch.

A series of experiments are described in which magnitude estimates of the perceived size of steel balls were made when the balls were actively rolled between the fingertip and several other body sites (thumb, thenar eminence, forearm). This movement, called scripting, involves actively moving an object by a touching surface over another surface of the body which is passively being touched. We define this active/passive activity as "intra-active touch" and the results show that the perceptual size of the balls is dependent upon the body part passively being activated. An additional series of experiments decoupled the actively generated and passively received tactile information by having subjects either perform the scripting on another individual's body site or by having the other individual roll the balls on the subject's various sites. The latter experiments showed that the passive body can contribute to the overall impression of the size of the balls, but only when the intra-active touching involved the glabrous skin of the hands. Intra-active touch between the active finger and the passively touched hairy skin of the forearm showed no effect of the touched surface on the perceived size of the balls. The results suggest that the mechanisms of intra-active touch are different when glabrous skin activates glabrous skin than when glabrous skin activates hairy skin.

Fingers↗
PubMed1992

Visual performance of pigeons following hippocampal lesions.

The effect of hippocampal lesions on performance in two psychophysical measures of spatial vision (acuity and size-difference threshold) was examined in 7 pigeons. No difference between the preoperative and postoperative thresholds of the experimental birds was found. The visual performance of pigeons in the psychophysical tasks failed to reveal a role of the hippocampal formation in vision. The results argue strongly that the behavioral deficits found in pigeons with hippocampal lesions when tested in a variety of memory-related spatial tasks is not based on a defect in spatial vision but impaired spatial cognition.

Animals↗
PubMed1998

Kinematic analysis of prehension movements in children.

The kinematics of the reach-to-grasp movement were analyzed in ten healthy children (age 6-7 years) under different experimental conditions: distance and size of the target objects, and visual feedback during the reach were varied in a within-subjects design. To assess age-related differences, the same experiments were performed in ten healthy adults. The experimental set-up was scaled according to body proportions to obtain equivalent conditions for both age groups. The temporal coupling between the transport and grasp components of prehension was very similar in children and adults. Peak transport velocity increased by the same factor in both age groups when the object distance was doubled. However, the decelerating approach phase was shorter in the children, who opened their hands relatively wider than adults. Unlike the adults, children failed to scale their grip aperture according to object size when visual feedback during the movement was lacking. The grip aperture increased with object distance in adults, but not in the children. The intrasubject variability of kinematic parameters was distinctly higher in the children. The results suggest that grip formation is not yet mature at an age of 6-7 years, depending more on visual feedback than in adult prehension.

Adult↗
PubMed1999

Tolerances of responses to visual patterns in neurons of the posterior inferotemporal cortex in the macaque against changing stimulus size and orientation, and deleting patterns.

Neuronal activities were recorded in areas TEO and TE of the inferotemporal cortex in four hemispheres of two monkeys during the performance of a visual pattern discrimination task. Tolerances of responses to patterns against changing stimulus size and orientation, and deleting patterns halves were investigated and compared between TEO and TE neurons. Of 311 neurons tested, 80 (26%) responded to one or more patterns out of four standard patterns. Of these 80 neurons, 50 (63%) were recorded in area TEO and 30 (38%) in area TE. Neurons responsive to patterns were recorded in both areas TEO and TE, however degrees of tolerance of responses were different between TEO and TE neurons. Tolerances of TEO neurons were moderate and degrees of tolerance varied from neuron to neuron. Responses to particular patterns were dependent on stimulus size, stimulus orientation, and/or completeness of patterns. By contrast, tolerances of TE neurons were generally strong. Responses to particular patterns were not affected by changing stimulus size, changing stimulus orientation nor deleting patterns halves. These results suggest that area TEO rather than area TE is involved in detecting and processing particular visual shapes.

Animals↗
PubMed1999

Integrated effect of stimulation at fixation points on EFRP (eye-fixation related brain potentials).

The purpose of this study was to investigate the integrated effect of stimulation at the fixation points just before and just after saccadic eye-movement (saccade) on eye-fixation related brain potentials (EFRP: P75 and N105). Checkerboard patterns were used as stimuli. In Experiment 1, changes in check sizes between two fixation points enhanced the amplitude of P75, while changes in the phases of patterns between the two points did not affect EFRP. This result showed that EFRP was affected by two fixation points, and that changes in the retinal image between the two points did not necessarily affect EFRP. In Experiment 2, the relationship between EFRP and check size was investigated in detail. A second order relationship between logarithm of check size and the latency of P75, and a linear relationship between logarithm of check size and the amplitude of N105 were found. The effect of check size on the amplitude of P75 which might explain the increased amplitude of P75 observed in Experiment 1 did not appear. These results suggest that EFRP might reflect relative higher processing than peripheral stimulation at one fixation point.

Adolescent↗