Vibrotactile adaptation fails to enhance spatial localization in adults with autism

Abstract

A recent study [Tannan, V., Tommerdahl, M., Whitsel, B.L., 2006. Vibrotactile adaptation enhances spatial localization. Brain Res. 1102(1), 109-116 (Aug 2)] showed that pre-exposure of a skin region to a 5 s 25 Hz flutter stimulus ("adaptation") results in an approximately 2-fold improvement in the ability of neurologically healthy human adults to localize mechanical stimulation delivered to the same skin region that received the adapting stimulation. Tannan et al. [Tannan, V., Tommerdahl, M., Whitsel, B.L., 2006. Vibrotactile adaptation enhances spatial localization. Brain Res. 1102(1), 109-116 (Aug 2)] proposed that tactile spatial discriminative performance is improved following adaptation because adaptation is accompanied by an increase in the spatial contrast in the response of contralateral primary somatosensory cortex (SI) to mechanical skin stimulation--an effect identified in previous imaging studies of SI cortex in anesthetized non-human primates [e.g., Simons, S.B., Tannan, V., Chiu, J., Favorov, O.V., Whitsel, B.L., Tommerdahl, M, 2005. Amplitude-dependency of response of SI cortex to flutter stimulation. BMC Neurosci. 6(1), 43 (Jun 21) ; Tommerdahl, M., Favorov, O.V., Whitsel, B.L., 2002. Optical imaging of intrinsic signals in somatosensory cortex. Behav. Brain Res. 135, 83-91; Whitsel, B.L., Favorov, O.V., Tommerdahl, M., Diamond, M., Juliano, S., Kelly, D., 1989. Dynamic processes govern the somatosensory cortical response to natural stimulation. In: Lund, J.S., (Ed.), Sensory Processing in the Mammalian Brain. Oxford Univ. Press, New York, 79-107]. In the experiments described in this report, a paradigm identical to that employed previously by Tannan et al. [Tannan, V., Tommerdahl, M., Whitsel, B.L., 2006. Vibrotactile adaptation enhances spatial localization. Brain Res. 1102(1), 109-116 (Aug 2)] was used to study adults with autism. The results demonstrate that although cutaneous localization performance of adults with autism is significantly better than the performance of control subjects when the period of adapting stimulation is short (i.e., 0.5 s), tactile spatial discriminative capacity remained unaltered in the same subjects when the duration of adapting stimulation was increased (to 5 s). Both the failure of prior history of tactile stimulation to alter tactile spatial localization in adults with autism, and the better-than-normal tactile localization performance of adults with autism when the period of adaptation is short are concluded to be attributable to the deficient cerebral cortical GABAergic inhibitory neurotransmission characteristic of this disorder.

Figure 1

Spatial localization tracking plots, under 2 conditions of adapting stimulus duration, were averaged for individual subjects. Top panel: Tracking data for two subjects from the control group, obtained in a previously reported study (Tannan et al., 2006). Note that spatial localization performance is distinctly better in the 5.0 sec vs. 0.5 sec duration adapting stimulus condition. Bottom panel: Tracking plots for each of the 4 subjects with autism.

Figure 2

Spatial localization under 2 conditions of adapting stimulus duration for adults with and without autism. Data displayed from the control subjects (left panel; previously reported (Tannan et al., 2006)), contrasts markedly from the data displayed in the middle panel which was obtained from observations of subjects with autism. Note that subjects with autism, although they clearly outperformed the controls in the 0.5 sec adapting condition, did not improve with the 5.0 sec adapting condition. Panel at far right summarizes the findings with averages of the last 5 trials.

Figure 3

Stimulus position and timing diagram of experimental protocol. A. A 2IFC tracking protocol was used to determine a subject’s spatial localization threshold under two conditions of adapting stimulus duration (0.5 or 5 sec). B. In each trial, the subject was instructed to report which of the two post-adapting stimuli was in the same place as the adapting stimulus. A correct/incorrect response resulted in a decrease/increase in the distance by 1 mm. Each run consisted of 20 trials. (Tannan et al., 2006)