Kim M. Goddard- kgoddard@acs.ucalgary.ca
Matthew I. Isaak
Elzbieta B. Slawinski
Department of Psychology
University of Calgary
Calgary, AB. CANADA
T2N 1N4
Popular version of paper 4pPP1
Presented Thursday afternoon, June 25, 1998
ICA/ASA '98, Seattle, WA
The notion that blind people are better "hearers" is a popular one. It seems intuitively reasonable to believe that when one sensory modality is compromised, other modalities are able to compensate. Yet, when researchers have investigated this phenomenon, little scientific evidence has emerged to support this idea. However, most of the studies that have looked at compensation, have focused on basic sensory acuity - for example, the ability to discriminate between two different frequencies or the ability to detect a silence between two identical frequencies (temporal resolution). Since the auditory system is already exquisitely designed to extract the basic information from most sounds in our everyday environment, it is unsurprising that researchers have been unable to find any behavioral evidence of enhanced auditory capabilities in a congenitally blind population. It is not clear what advantage would be conveyed to the recipient of such a sensory enhancement.
Nevertheless, evidence has been accumulating that suggests that physiological changes within the brain occur in a number of different areas and in a number of different species of visually deprived animals. Assuming that these physiological changes also characterize congenitally blind humans, we found it difficult to believe that these structural changes were in no way associated with any functional behavioral changes. Perhaps the sensory level was the wrong place to look for evidence of behavioral auditory compensation. We wondered if compensation could be occurring at so-called higher levels of auditory processing, where simple acoustical attributes such as frequency, intensity or duration have already become "transformed" into auditory cognitive representations. By this we mean that the physical properties of any given sound have been encoded to the point where the sound in question has become a meaningful piece of information that could be acted upon. So, for example, the frequency components of a speech sound have been processed by the auditory system to the extent that it could be responded to as the vowel "a", as opposed to the vowel "e". It was with this working definition of cognitive compensation in mind that we embarked on the present investigation.
This study examined cognitive auditory compensation from an attentional/temporal processing perspective. We employed a task that involves a series of auditory streams consisting of rapidly presented pure tones (approximately 11 tones/second). Within each stream, one or two tones are louder than the rest and the subject's task is to identify the louder tones (targets) according to pitch (low, medium or high). The interval between the louder tones is systematically manipulated. Similar analogue tasks have been used extensively in vision research and results from these types of experiments show that when the first target is attended to and subsequently identified, the ability to identify the second target is impaired at short intervals, typically less than 500 ms. Interestingly, when the first target is actively ignored, the ability to identify the second target is not impaired. Ultimately then, these types of tasks measure the cognitive consequences of attention in terms of the ability to identify a second stimulus after a first has been attended to and how this changes as a function of time.
We hypothesized that the congenitally blind would perform better on the task. We reasoned that if indeed there was such a possibility of cognitive compensation, then perhaps their efficiency in allocating attention to a target and subsequently identifying it might be enhanced relative to a non-blind person. However, we also wanted to make sure that there was no compensation at the sensory level, or in other words, that this population was not just simply faster at processing sounds in general. Thus, we also included a measure of temporal resolution. Temporal resolution (or temporal acuity) refers to the ability of the auditory system to respond to rapid changes in the envelope of a sound over time. If the system is slow (i.e. temporal resolution is poor), then the internal representation of the sound will not faithfully represent the temporal changes present in the physical stimulus - analogous to a blurry, out-of-focus image. Temporal resolution is typically measured by using a gap detection technique, which was the technique employed in this experiment.
This study found no differences in temporal resolution between the blind and the non-blind. This confirmed the findings from other studies that have found no evidence of behavioral compensation at the sensory level. In contrast however, our congenitally blind did perform better on the cognitive attentional task, at least at very short intervals (less than 180 ms). We have interpreted this finding as tentative evidence for behavioral compensation at the cognitive level. It is presumed that, for blind people, there is particularly mportant information contained in auditory signals at these short intervals and in which an enhanced ability to attend to and extract this information would be beneficial in the day-to-day cognitive negotiation of their environment. Finally, we note parenthetically that speech signals contain crucial information within these temporal windows such as initial fundamental frequencies and formant transitions. We speculate that the special reliance of this population on verbal interactions (i.e. speech perception) with others may contribute to this enhanced ability. We would further speculate that, given these enhanced attentional abilities, and to the extent that they subserve other information-extracting auditory processes, the congenitally blind may possibly possess superior sound localization abilities.