Perception

The sensory system of animals, the words and language that connect the thoughts of one individual with those of another, and man-made communication systems, are all greatly influenced by the way information is coded. It is suggested that the role of inductive reasoning is to improve the efficiency of linguistic communication by changing the code, and its effectiveness in performing this function explains its survival value—the reason why this habit of mind is biologically successful and persists. The principles of coding are best understood in a limited, well-defined situation, but inductive reasoning should be viewed in a broad context, which must include the physiological and psychological mechanisms that form perceptions from physical stimuli and attach words to perceptions.

Two conditions of exposure to prism rearrangement were used to explore specificity of visuo—motor adaptation. In one condition subjects viewed, in sequential alternation of one minute periods, the movements of one arm through base right prisms and those of the other arm through base left prisms for a total of 20 min. In the second condition subjects viewed the movements of only one arm for one minute periods followed by closing the eyes for the next, alternating between viewing arm movements and closing the eyes for a total of 20 min. The magnitude of postexposure sensorimotor changes following the alternating limb-exposure condition was equivalent and opposite in direction in each limb. Unilateral exposure generated commensurate sensorimotor changes in responses entailing the exposed arm. These results show that prism-mediated visual reafference from arm movements acts to recalibrate target-directed orientations of that arm only, and does not interact, or interfere with, recalibration of directed orientations of the other limbs.

It is now well established that the cortical neurones of cats and monkeys respond best to either lines or edges of a specific orientation. The respective tuning curves of these cells can be broadened by rearing the animals in abnormal visual environments during their critical period of development.

Since neurones in the human visual system probably also possess this orientational selectivity, then it is not inconceivable that humans also could demonstrate similar orientational selectivity loss if they too have an anomalous visual input during their early years of life. To test this hypothesis, a simultaneous subthreshold masking technique is proposed as a means for assessing psychophysically the selectivity properties within the human striate cortex. Both visually normal and abnormal systems were considered.

No selectivity loss is demonstrated for either the orientationally deprived (i.e. high astigmats) or monocular form deprived (high anisometrope) subjects. The effect of large eye movements (nystagmus) does, however, bring about a coarser selectivity. These results are discussed in relation to recent neurophysiological studies of the critical period in lower vertebrates.

Five subjects adapted for 30 min to a textured disc lying in front of the fixation point with 0·1 deg⁽¹⁾ crossed disparity, which rotated clockwise at 4 rev/min, alternating with a disc behind the fixation point, with 0·1 deg of arc uncrossed disparity, which rotated anticlockwise. A stationary test field then appeared to rotate anticlockwise when it lay in front of the fixation point, and clockwise when it lay behind. Conversely, a test field in the plane of fixation briefly appeared to lie a few millimetres behind the fixation plane when it rotated clockwise, and in front when it rotated anticlockwise.

The movement aftereffect contingent on disparity reappeared each time the test disparity was reversed, but the total duration of each successive aftereffect in the series decreased exponentially with elapsed time. Movement aftereffects contingent on disparity were very much stronger than those contingent on colour and won out over them when disparity was pitted against colour.

Rotation of a grating at around 2π rad s⁻¹ in continuous illumination results in the appearance of a band lying almost perpendicular to the lines. Two possible bases for the occurrence of this effect were examined. Firstly, the band might represent a moiré fringe due to the overlap of the present grating with one persisting in the visual system from a previous position. Secondly, those parts of the lines farthest from the bisecting diameter will have the greatest linear displacement during rotation; thus, the extremities of each line might fail to be resolved, while those parts nearer the centre are adequately resolved, resulting in the appearance of a band. Three experiments were conducted to test predictions from the two hypotheses. The first experiment involved manipulation of the spatial frequency of the square-wave gratings and the size of the discs bearing them. In the second the line spacing was varied while the line width was kept constant. In the third experiment the angular velocities required to produce bands of different widths were determined. The results did not support either hypothesis unequivocally, and the possibility that both are involved in the effect cannot be discounted.

Observation of the rotating gratings under stroboscopic illumination produced either moiré fringes or complex doily-like patterns, depending upon the angular velocity and strobe frequency. These ‘overlap’ effects were used to determine the duration of visual persistence in a fourth experiment. At certain combinations of high angular velocities and strobe frequencies, apparent rotation effects are visible simultaneously in opposite directions.

Ascending series detection and recognition thresholds, both with and without interstimulus water rinses, and pair comparison thresholds, with varied presentation order, were measured for NaCl. Thresholds were measured in situ against a normal salivary background and also after rinses with 60 mM and 1 M NaCl. Predicted changes in the level of adaptation of the taste receptors, due to NaCl concentration changes in the mouth, were seen to vary the measured thresholds. Ascending series thresholds without interstimulus rinses were highest of all, although they were reduced by rinsing. The order of presentation in pair comparison tests was seen to vary thresholds in a manner predicted by proposed adaptation levels. Effects were also noted which were due to criterion definition and language. The trends found in the present experiments were also found in the literature.

The detectability of one part of an illuminated field which has a straight edge as the border to the remaining part of the field was studied experimentally while the illumination of the total field had an irregular contrast based on a gaussian-noise spatial structure. The two parts of the field had different average illuminations. The detectability of the edge was studied for various illumination differences, field areas, edge lengths, and sharpnesses. The results are discussed in terms of the characteristics of the background noise and the visual system. The observations are interpreted in terms of a model that assumes spatial differentiation and spatial integration.

Errors in vertical settings of a test rod occur when the rod is enclosed in a laterally-tilted square-outline frame. The majority of previous experiments which have investigated this rod-and-frame effect have used a single frame tilt, usually 28°, and have tabulated errors as average unsigned deviations from gravitational vertical. Evidence is presented that, when the illusion is measured by taking algebraic differences between constant (signed) errors made with and without the frame being present, illusions occur in the direction of frame tilt for frame tilts up to about 25° from vertical (repulsion effects) but that directionally opposite illusions (attraction effects) occur for frame tilts between 25° and 45°. At the frame tilts used most frequently in previous studies (25° to 30°) little or no illusion occurs. A distinction is drawn between the rod-and-frame illusion (RFI), which has an angular function similar to the simple tilt illusion and aftereffect, and the rod-and-frame test (RFT), which uses unsigned deviations from vertical as its measure of error and which probably bears little or no relationship to the RFI.

Solutions of sodium chloride, sucrose, tartaric acid, and quinine sulphate were each presented in successive ascending series, both with and without interstimulus tapwater rinses, to six subjects. A tendency for detection and recognition thresholds to be raised on successive ascending series was noted for all stimuli. The effects were explained in terms of adaptation to stimulus residuals. When water rinses were taken between tastings, significantly lower thresholds were noted for sodium chloride and glucose. Although this trend did occur for tartaric acid and quinine sulphate, it was not significant. The literature was examined and seen to show similar trends.

Two experiments employed dichotic listening and a visual analogue of dichotic listening. Detection of signals on a nonattended channel deteriorated for both modalities as the signal rate increased. However, deterioration in the primary task, vocal shadowing, was greater for auditory presentations. A third study showed that modality differences disappeared when shadowing difficulties were equated. In addition, it was argued from experiments 1 and 2 that increasing signal rate led to an increase in the selectivity of attention.

At the point where shadowing began to deteriorate there was evidence of reciprocity or trading relations between primary and secondary tasks.

Previous studies of a difficult reasoning problem have elicited inconsistent utterances. This study tested the hypothesis that the inconsistency is due to an asymmetrical encoding of the problem between the two hemispheres in the brain. The hypothesis was confirmed.