Research report
Mental rotation of tactile stimuli

https://doi.org/10.1016/S0926-6410(02)00063-0Get rights and content

Abstract

When subjects decide whether two visual stimuli presented in various orientations are identical or mirror-images, reaction time increases with the angular disparity between the stimuli. The interpretation of this well-known observation is that subjects mentally rotate images of the stimuli until they are in congruence, in order to solve the task. Here we review studies involving mental rotation of tactile stimuli. Mental rotation in tactile tasks is specifically associated with the requirement for mirror-image discrimination, as opposed to identity judgments. The key brain region mediating mental rotation of tactile stimuli seems to be the parietal cortex. Visual processing appears to facilitate task performance. We report an experiment from our laboratory addressing the nature of the reference frame for mental rotation of tactile stimuli. Our observations indicate that when the hand is directly in front of the body, with the head facing forward, the shortest reaction times for mirror-image discrimination of stimuli applied to the fingerpad are obtained when the longitudinal axis of the stimulus is in or parallel to the sagittal plane, even when this is perpendicular to the long axis of the finger. Thus, the reference frame for mental rotation of tactile stimuli is not purely hand-centered. This is consistent with other findings indicating variable assignment of reference frames for tactile perception.

Introduction

Shepard and Metzler [43] first presented evidence for the cognitive process of mental rotation. Their subjects viewed drawings of variously oriented three-dimensional objects, consisting of cubes stacked face-to-face. The task was to decide if the two objects presented in each trial were identical or mirror-images of each other. Subjects reported turning the objects in their minds to accomplish the task. Moreover, reaction times (RTs) rose linearly with increased angular disparity between the two objects depicted. Both the RT data and the introspective reports were consistent with the notion that subjects formed mental images of the objects and rotated one of them at a fixed rate until it was in congruence with the other. Hence, a new paradigm for studying complex mental processing emerged. Evidence of mental rotation, in the form of monotonically increasing RT as a function of angular disparity of the stimuli, was subsequently found in visual tasks using stimuli other than the originally used cube-assemblies [43], such as alphanumeric characters [13], [47] and pictures of the body, hands and feet [3], [29], [30]. Mental rotation has been evoked both during tasks requiring one stimulus to be compared against some previously memorized standard [3], [13], [29], [30], [47] and in tasks where two stimuli are to be compared against each other [43]. Explicit instructions to use a mental rotation strategy produced the same general RT profile as tasks where no such instruction was given [1], [12], [24], [37]. Almost all these tasks required mirror-image discrimination of the stimuli.

Studies using functional neuro-imaging and event-related potentials (ERPs) have shown that mental rotation of visual stimuli activates areas of occipital, frontal, and parietal cortex. The rotation process itself, usually isolated through subtraction of activity due to mirror-image discrimination (at zero angular disparity) from rotation task activity or by the analysis of particular epochs of the trial during which mental rotation occurs, seems to particularly involve parietal cortex [2], [3], [12], [17], [24], [31], [33], [37], [46], [47]. Other cortical areas are also implicated. Specifically, inferior parietal cortex, postero-superior parietal cortex, mid-ventrolateral frontal cortex, medial frontal cortex and extrastriate occipital cortex have been identified as being selectively more active during the mental rotation process than the mirror-image discrimination process [2], [3], [12], [24], [31], [47]. Mental rotation studies presenting visual depictions of hands also report extensive activity in motor regions including primary motor, premotor and supplementary motor cortex, basal ganglia and cerebellum [3], [24], [31]. This is consistent with the idea that subjects imagine rotating their own body parts when presented with such stimuli, but imagine rotating an external object when presented with other kinds of stimuli. Activity has been reported in the head of the caudate nucleus [2], and in premotor and primary somatosensory cortex [12] during some mental rotation tasks presenting visual stimuli that do not represent body parts, raising the possibility of somatic sensorimotor imagery even for such stimuli.

These studies offer insight into the mental rotation of visually presented stimuli. An interesting question is to what extent the findings reflect modality-specific transformations. Does mental rotation depend upon processing in visual regions of the brain? Does the neural basis for mental rotation differ between modalities? To address these issues, mental rotation has been investigated using tactile stimuli. The following is a brief review of studies of this sort. In the remainder of the article, mental rotation of cutaneous stimuli under both passive and active (haptic exploration) conditions will be referred to as ‘tactile mental rotation’. At the end of the article, we present an experiment from our laboratory that addresses the nature of the reference frames used in mental rotation of tactile stimuli.

Section snippets

Stimuli

A number of different tactile stimuli have been utilized in studies of tactile mental rotation. Although the stimuli were three-dimensional, the relevant stimulus information was generally contained in only two dimensions. For example, the tactile stimuli used by Marmor and Zaback [25] were flat geometric shapes that resembled an ice cream cone with a bite out of one side of the scoop. The objects had a uniform thickness, so that the relevant shape information was contained in the other two

Brain regions involved

ERP studies have provided the best evidence to date for the neural basis of tactile mental rotation. Röder et al. [36] tested normal subjects using patterned tactile displays composed of discrete circular elements (resembling Braille dots). Their task design allowed them to separate stimulus encoding from mental rotation. In the first phase of the trial, subjects explored a tactile stimulus for a finite period of time. Then, after an auditory cue, the subjects mentally rotated the memorized

Dependence of tactile mental rotation on visual processing

Subjects engaging in visual mental rotation tasks report rotating the objects in their minds [43]. As there can be a strong visual component to this experience, researchers sought to determine if mental rotation depends on the visual modality or simply on a general, non-visual process of spatial imagery. While the ERP studies discussed above [36], [38] implicate parietal but not occipital cortex in tactile mental rotation in normal subjects, this does not rule out the possibility that tactile

Reference frames of tactile stimuli

When one perceives a tactually presented pattern, one assigns to it certain axes, to enable characterizing its top and bottom, front and back, left and right, etc. These axes create a frame of reference within which the tactile pattern is interpreted. This assignment process is complex and may depend on a number of different spatial factors, causing tactually applied letters to be perceived as mirror-reversed under certain conditions. For example, Oldfield and Phillips [28] reported that

Current investigation: does tactile mental rotation use a hand-centered coordinate system?

We carried out an experiment in our laboratory to address the nature of the reference frame within which tactile stimuli in the horizontal plane are mentally rotated. A preliminary report of the experiment has been presented [8].

Conclusions

The similar results of tactile and visual mental rotation studies suggest that mental rotation is a process which depends upon spatial imagery and analysis that need not be modality-specific. However, the fact that sighted and adventitiously blind subjects are superior to the congenitally blind suggests that there is a distinct advantage to using visual processing or imagery in mental transformation of tactile stimuli. Moreover, it is clear that tactile stimuli are often interpreted, not in

Acknowledgements

This work was supported by a grant to KS from the National Eye Institute (RO1 EY 12441) and by an ARCS award to SCP.

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