Event-related brain potentials reveal age-related changes in parafoveal-foveal integration during sentence processing

Abstract

Normative aging is associated with deficits in visual acuity and cognitive control that impact the allocation of visual attention, but little is known about how those changes affect information extraction and integration during visual language comprehension in older adulthood. In the current study, we used a visual hemi-field flanker RSVP paradigm with event-related brain potentials to study how older readers process fine-grained aspects of semantic expectancy in parafoveal and foveal vision. Stimuli consisted of high constraint sentences with expected, unexpected but plausible, or anomalous parafoveal target words, as well as low constraint sentences with neutral but expected target words. Older adults showed graded parafoveal N400 effects that were strikingly similar to younger readers, indicating intact parafoveal semantic processing. However, whereas young adults were able to use this parafoveal pre-processing to facilitate subsequent foveal viewing, resulting in a reduced foveal N400 effect, older adults were not able to. Instead, older adults re-processed the semantics of words in foveal vision, resulting in a larger foveal N400 effect relative to the young. Collectively, our findings suggest that although parafoveal semantic processing per se is preserved in aging, there exists an age-related deficit in the ability to rapidly integrate parafoveal and foveal visual semantic representations.

Keywords: Aging; ERP; Parafoveal processing; Sentence processing.

Figure 1

(a) Grand average event-related brain potentials at representative electrode sites. The time window encompasses both parafoveal and foveal viewing for high constraint sentences with expected, unexpected, and anomalous target words and target words in low constraint contexts. Time 0 indicates when the critical target appeared in parafoveal vision. This is followed at 450 ms by the subsequent triad, in which the target appeared in foveal vision, marked in the diagram as “foveal onset”. (B) Scalp distribution of the effect of expectancy (unexpected – expected) and congruity (anomalous – expected) violations on parafoveal and foveal N400 responses and on the late positivity following foveal viewing.

Figure 2

Results of individual item-level N400 amplitude by cloze probability and foveality, overlaid with the slope of the cloze effect for the parafoveal N400 (red) and foveal N400 effect (blue). Note that negative is plotted up, for ease of comparison with the ERP plots. N400 amplitudes were continuously graded with respect to cloze probability to a similar degree in both foveal and parafoveal vision.

Figure 3

Age-group comparison of foveal and parafoveal N400 effects. a) Long grand -average ERPs at a representative midline parietal electrode as a function of target word expectancy and Age. b) Difference wave of the canonical N400 congruity violation (Anomalous vs Expected) overlaid separately for young and old. c) Age-group comparison of the mean N400 congruity violation effect in foveal and parafoveal vision. Young group data original reported in Stites et al. (2017).

Figure 4

Results of the mass-univariate test of the congruity and expectancy violations in parafoveal and foveal vision in older adults. Raster-style heat-map plots the local FDR controlled one-tailed t-tests of the expectancy and congruity violation effects. Results are presented in 20-ms bins beginning around the offset of the foveal N400 and extending until the end of the epoch. Anterior electrodes are depicted in the upper portion of the figure, medial electrodes are presented in the center, and posterior electrodes are presented in the lower portions of each panel. Significant positive t-tests are plotted in red.