Do Alterations in Low-Level Visual and Auditory Processing Co-Occur in Autistic Individuals?

Background:  Multiple behavioural studies have demonstrated enhanced processing of low-level auditory (eg: pitch discrimination) and mid-level visual (eg: pattern detection) information in autism. The enhanced perceptual functioning model proposes that such differences originate from a combination of enhanced bottom-up feed forward flow of perceptual information combined with a superior independence from downward streams in both visual and auditory modalities. At the cortical level, it has been suggested that differences in elementary information processing result from altered local neural connectivity characterized by atypical inhibitory/excitatory lateral interactions. This would result in increased spatial-frequency tuning mediated by mechanisms within primary visual areas in autism (Bertone et al., 2005). This idea has been supported in the visual domain (Keita et al., 2011; Vandenbrouke et al., 2008) and predicts the same type of alteration within the auditory domain. At a physiological level, the minicolumns theory (Casanova et al., 2002) also proposes a superior ability to discriminate low-level information across sensory modalities. Despite these multiple theories and predictions, it remains unknown whether such locally-oriented perception co-occurs in both visual and auditory domains in autism.

Objectives:  To establish the prevalence of autistic individuals who present with specific enhanced low-level auditory and visual perceptual abilities, then to determine whether these abilities co-occur in autistics and matched typically developing peers.

Methods:  43 autistics and 38 controls (ages 14 to 35) were tested on three psychophysical tasks: 1) pitch discrimination, 2) contrast discrimination of luminance-defined gratings, and 3) visual spatial frequency (SF) discrimination. Participants were matched at the group level on Raven’s Progressive Matrices and analyses controlled for differences in Wechsler FSIQ. All participants had normal or corrected-to-normal visual and auditory acuity.

Results:  Although no between-group differences were manifested for contrast and SF sensitivity (F(1,77)=.332, p=.566 and F(1,71)=.001, p=0.972 respectively), between-group mean comparisons showed that autistics were significantly more sensitive than controls in detecting small pitch variations (F(1,64)=18.838, p<.001). A peak prevalence analysis, using a modified t-test (Crawford & Howell, 1998), revealed enhanced pitch and low-level visual sensitivity (≥1 SD control mean) in 33% and in 7% of autistics, respectively. Within autistics with enhanced pitch sensitivity, 58% also displayed a Block Design score significantly above their Wechsler FSIQ.

Regression analyses, incorporating a group X predictor interaction factor, tested for between-group differences. Results showed that pitch sensitivity predicted SF and contrast discrimination thresholds differently for both groups (SF:p=.024; Contrast:p=.011). Pitch sensitivity was a significant predictor of both SF (p=.018 R²=.229) and contrast sensitivity (p=.006 R²=.227) in autistics, but not in controls (p=.702 R²=.005 and p=.856 R²=.001 respectively).

Conclusions: Enhanced pitch sensitivity is present in one third of autistics and is more common in those who display visuo-spatial abilities significantly above what is predicted by their Wechsler FSIQ. More importantly, the results demonstrate a distinct relationship between low-level auditory and visual perception in autism that was not found for control participants. The fact that pitch sensitivity significantly predicts low-level visual abilities in autistics, but not in non-autistics, suggests that atypical perceptual processing across modalities in autism may arise from common underlying mechanisms.