Effort Not Speed Characterizes Comprehension of Spoken Sentences by Older Adults with Mild Hearing Impairment

Abstract

In spite of the rapidity of everyday speech, older adults tend to keep up relatively well in day-to-day listening. In laboratory settings older adults do not respond as quickly as younger adults in off-line tests of sentence comprehension, but the question is whether comprehension itself is actually slower. Two unique features of the human eye were used to address this question. First, we tracked eye-movements as 20 young adults and 20 healthy older adults listened to sentences that referred to one of four objects pictured on a computer screen. Although the older adults took longer to indicate the referenced object with a cursor-pointing response, their gaze moved to the correct object as rapidly as that of the younger adults. Second, we concurrently measured dilation of the pupil of the eye as a physiological index of effort. This measure revealed that although poorer hearing acuity did not slow processing, success came at the cost of greater processing effort.

Keywords: aging; cognitive effort; eye tracking; hearing loss; pupillometry; speech comprehension.

Figure 1

Experimental stimuli and procedures. (A) Waveform of an example sentence showing the knowledge point (KP) based on a cloze procedure, the relative times after the KP that participants’ eye-fixation indicated knowledge of the target object (eye fixation time; EFT), and when the target picture was selected with the computer mouse (overt response time; ORT). (B) An example picture array. Depicted in the bottom left corner is the target picture (key), while the other three pictures represent unrelated lures.

Figure 2

Results for gaze time and overt responses. Two vertical bars on the left show mean latencies from the KP in a sentence to the time point when younger and older adults’ eyes fixated longer on the target picture than on the lures (EFT). Two vertical bars on the right show the mean latency from the KP in a sentence to the selection of the correct target picture with a computer mouse (ORT). Error bars are one standard error. ***Significant pairwise differences, p < 0.001.

Figure 3

Better-ear pure-tone thresholds from 500 Hz to 4000 Hz for the three participant group. Hearing profiles for individual listeners within each participant group are shown in color, with the group average drawn in black. The shaded area in each of the panels indicates thresholds less than 25 dB HL (the range considered clinically normal for speech; Katz, 2002).

Figure 4

Mean adjusted pupil diameter leading up to the moment of comprehension. Pupil diameters calculated over a 1-s window preceding participants’ eye fixations on the target picture. Data are shown for younger adults (left vertical bar), older adults with normal hearing acuity (middle vertical bar), and older adults with hearing impairment (right vertical bar). Error bars are one standard error. **Significant pairwise differences, p < 0.01.