Vision merges with touch in a purely tactile discrimination

To construct a coherent percept of the world, the brain continuously combines information across multiple sensory modalities. Simple stimuli from different modalities are usually assumed to be processed in distinct brain areas. However, there is growing evidence that simultaneous stimulation of multiple modalities can influence the activity in unimodal sensory areas and improve or impair performance in unimodal tasks. Do these effects reflect a genuine cross-modal integration of sensory signals, or are they due to changes in the perceiver's ability to locate the stimulus in time and space? We used a behavioral measure to differentiate between these explanations. Our results demonstrate that, under certain circumstances, a noninformative flash of light can have facilitative or detrimental effects on a simple tactile discrimination. The effect of the visual flash mimics that produced by a constant tactile pedestal stimulus. These findings reveal that sensory signals from different modalities can be integrated, even for perceptual judgments within a single modality.     

Amplitude and spatial-period discrimination in sinusoidal gratings by dynamic touch

We measured discrimination thresholds for sinusoidal gratings using active dynamic touch. In the first experiment we measured the thresholds for amplitude discrimination as a function of amplitude and spatial period. Thresholds ranged between 10.8%, and 15.8% of the standard amplitude. We showed that amplitude differences as small as 2 microm can be detected. We found that Weber fractions for amplitude discrimination are constant over a range of amplitudes, but are influenced by the spatial period of the grating; discrimination improved with increasing spatial period. In the second experiment we determined the thresholds for spatial-period discrimination. We used the same design as in the first experiment. Weber fractions ranged from 6.4% to 11.8%. Amplitude was found to have no effect on the Weber fractions for spatial-period discrimination. However, the spatial period had an effect on the Weber fractions: larger spatial periods yielded lower Weber fractions.     

Multisensory integration of vision and touch in nonspatial feature discrimination tasks

Multisensory integration of nonspatial features between vision and touch was investigated by examining the effects of redundant signals of visual and tactile inputs. In the present experiments, visual letter stimuli and/or tactile letter stimuli were presented, which participants were asked to identify as quickly as possible. The results of Experiment 1 demonstrated faster reaction times for bimodal stimuli than for unimodal stimuli (the redundant signals effect (RSE)). The RSE was due to coactivation of figural representations from the visual and tactile modalities. This coactivation did not occur for a simple stimulus detection task (Experiment 2) or for bimodal stimuli with the same semantic information but different physical stimulus features (Experiment 3). The findings suggest that the integration process might occur at a relatively early stage of object-identification prior to the decision level.     

Factors of hemifield differences in form discrimination

The influence of some process-limiting variables on hemifield differences for nonsense shapes in a "same-different" classification task was examined. "Different" responses, but not "same," did show systematic effects. Weakly discriminable and unfamiliar shapes which were presented successively yielded a significant left-hemifield advantage, while highly discriminable and more familiar shapes presented simultaneously yielded a nonsignificant right-hemifield advantage. Discriminability, considered independently of the other variables, had effects in opposite directions: low discriminability tended to favor more the right than the left hemifield in each experiment, but the right hemifield more than the left when the comparison was made between experiments. The results are discussed in terms of the spatial-frequency model (Sergent, 1982b, 1983) and the analytic-holistic distinction.     

Frequency discrimination between and within line gratings by dynamic touch

Line gratings were used to investigate the tactual discrimination thresholds for line frequency. In Experiment 1, participants were asked to discriminate between two gratings, each with a different line frequency. We used four standard frequencies in the eightfold range from 0.5 to 4 lines/cm. Thresholds were found to be constant at about 10.6%. In this experiment, we also measured hand speed and contact force. Hand speed was roughly in the range between 0.12 and 0.44 m/sec; contact force ranged from 0.62 to 2.76 N. In Experiment 2, we determined discrimination thresholds for line frequency transitions within a single grating. We used two frequencies and three transition lengths. The transition length had no effect on the threshold. In a third experiment, line frequency was modulated periodically. Varying the standard frequency and the size of the modulation period was found to have no effect on the discrimination thresholds. We conclude three things. First, Weber fractions for line frequency discrimination decrease as a function of line frequency within the experimental range. Second, discrimination thresholds are not altered by the length of the transition between two adjacent gratings with different line frequencies. And finally, the size of a modulation period in periodically modulated gratings is of no influence on the modulation detection threshold.     

Visual form discrimination: Multidimensional analyses

Nonmetric multidimensional scaling techniques were used to evaluate discrimination latency as a similarity measure. A three dimensional Euclidian solution was found for a sample of random polygons. The dimensions were labeled compactness, jaggedness, and elongation. The spatial configuration was highly similar to results obtained from similarity estimates and the dimensions were systematically related to physical shape measures.     

A developmental study of form discrimination in pre-school children

Using a matching-from-sample technique the discriminative ability of a group of pre-school children was tested twice, at mean ages 3-8 and 4-9, in relation to an independent test of their comprehension of the notion “same”. A sequence of three stages is described, for both shape and orientation discrimination; in the first, characterized by a large number of errors, the child appears to respond largely in terms of proximity of a comparison figure to the standard; in the second his responses reflect gross aspects of similarity between the figures in the comparison set, in that several matches are made to one standard all of which have certain attributes in common with it. Finally correct and unique choices are made. Attributes used in differentiating figures are described, and a number of theoretical and methodological problems are discussed.     

Detection versus discrimination of visual orientation

The role of focused attention in vision is examined. Recent theories of attention hypothesize that serial search by focal attention is required for discrimination between different combinations of features. Experiments are reported which show that the mixture of a few (less than five) horizontal and vertical line segments embedded in an aggregate of diagonal line segments can be rapidly counted (also called 'subitizing') by a parallel (preattentive) process, while the discrimination between horizontal and vertical orientation requires serial search by shifting focal attention to each line segment. Thus detecting and counting targets that differ in orientation can be done in parallel by a preattentive process, whereas knowing 'what' the orientation of a target is (horizontal or vertical, ie of a single conspicuous feature) requires a serial search by focal attention.     

Facilitation versus inhibition in non-spatial attribute discrimination tasks

Inhibition of Return is a delay in initiating attentional shifts to previously inspected locations. It has been explained as a mechanism to facilitate visual search of a scene by inhibiting the allocation of attention to locations that have already been examined. We (Hu, Samuel, &; Chan, Journal of Experimental Psychology: General, 2010) recently demonstrated that similar processing costs can appear when a non-spatial attribute (color or shape) repeats—detection of a target stimulus was slower if the target shared color or shape with a recently presented cue. In the current study, we test whether such inhibitory effects occur for non-spatial attribute repetition when observers must make a discrimination judgment about targets. We found two independent effects: First, there was a standard location-based IOR effect—target discrimination was slower when the target appeared in the same location as a preceding cue. Second, reaction times were faster if the target's color or shape matched the cue's color or shape; this facilitation effect contrasts with both the location-based inhibition that was present in the current experiments, and with the inhibitory effect of feature repetition in our previous detection task study. The data are best accounted for by a three-factor model recently suggested by Lupiáñez (Attention and time, 2010).