Optimally efficient neural systems for processing spoken language

Abstract

Cognitive models claim that spoken words are recognized by an optimally efficient sequential analysis process. Evidence for this is the finding that nonwords are recognized as soon as they deviate from all real words (Marslen-Wilson 1984), reflecting continuous evaluation of speech inputs against lexical representations. Here, we investigate the brain mechanisms supporting this core aspect of word recognition and examine the processes of competition and selection among multiple word candidates. Based on new behavioral support for optimal efficiency in lexical access from speech, a functional magnetic resonance imaging study showed that words with later nonword points generated increased activation in the left superior and middle temporal gyrus (Brodmann area [BA] 21/22), implicating these regions in dynamic sound-meaning mapping. We investigated competition and selection by manipulating the number of initially activated word candidates (competition) and their later drop-out rate (selection). Increased lexical competition enhanced activity in bilateral ventral inferior frontal gyrus (BA 47/45), while increased lexical selection demands activated bilateral dorsal inferior frontal gyrus (BA 44/45). These findings indicate functional differentiation of the fronto-temporal systems for processing spoken language, with left middle temporal gyrus (MTG) and superior temporal gyrus (STG) involved in mapping sounds to meaning, bilateral ventral inferior frontal gyrus (IFG) engaged in less constrained early competition processing, and bilateral dorsal IFG engaged in later, more fine-grained selection processes.

Keywords: cohort model; inferior frontal gyrus; lexical competition; lexical selection; spoken word recognition.

Figure 1.

Results of behavioral study. (A) Item-wise pre-nonword point duration plotted against RT. (B) Mean lexical decision RTs at early, middle, and late nonword points.

Figure 2.

Behavioral data in the scanner. Pre-nonword point duration plotted against RT (with post-nonword point duration covaried out).

Figure 3.

Significant activation for the contrast of nonwords minus baseline (silence) at a threshold of P < 0.005, voxel-level uncorrected, and P < 0.05, cluster-level corrected. Color scale indicates t-value of contrast.

Figure 4.

(A) Increasing activation for later nonword points at a threshold of P < 0.005, voxel-level uncorrected, and P < 0.05, cluster-level corrected. (B) The mean activation value of the significant cluster in (A) for each nonword point condition.

Figure 5.

(A) Significant activation for increasing cohort competition (red) and increasing cohort selection (green) at a threshold of P < 0.005, voxel-level uncorrected, and P < 0.05, cluster-level corrected. (B) The cohort competition and selection effects rendered at a lower threshold of P < 0.01, voxel-level uncorrected, and P < 0.06, cluster-level corrected.