fMRI Reveals the Role of the Left Anterior Fusiform Gyrus in Letter Detection and Identification

Using event-related fMRI, we investigated the neural correlates of letter detection and identification. Participants were required to detect or identify a single target letter embedded in noise. Prior to experimental runs, psychophysical contrast thresholds for letter detection (“Is a letter present or absent?”) and for identification (“If a letter is present, which letter is it?”, 8 letter choice) were determined. Experimental runs were divided into detection and identification runs and letters were presented in random order at four levels of contrast for both detection and identification. The four levels of contrast were based on individual threshold measurements and included two below threshold levels, far below (FB) and just below (JB), and two above threshold levels, far above (FA) and just above (JA). The FA contrast for detection was also the JA contrast for identification, and the JA contrast for detection was also the JB contrast for identification. A noise-only stimulus (N) with no embedded letter was included as a control. In the left anterior fusiform gyrus (LAF), a region known to be involved in letter processing, letters above detection threshold (FAD and JAD) produced more activation than letters below detection threshold (FBD and JBD). FBD and JBD letters did not differ from each other and also did not differ from the N stimulus. In contrast, FAD letters, that were also just above identification threshold, produced more activation than JAD letters, that were below identification threshold. Thus, we found that activation in the letter selective LAF region is driven only by detectable letters, and responds in a graded fashion to detectable letters depending on whether or not the letters are identified.