The visual world is highly dynamic: Visual scenes are constantly changing as a result of the movement of objects and fast and frequent eye movements. Observers must therefore accurately and reliably assign identity to rapidly changing events. In adults, the ability to perceive and individuate changing events is limited to rates no faster than 7 to 10 Hz. This limit has been taken as a measure of the temporal resolution of visual attention (Battelli et al., 2001
; Verstraten et al., 2000
). The present study is the first to characterize the temporal resolution of visual attention in infants, as well as its developmental course.
We measured temporal frequency thresholds at which an out-of-phase flickering stimulus could be identified by infants ages 6 to 15 months. Our data establish that the resolution of temporal visual attention is strikingly poor in infants: Six- and 9-month-olds could individuate alternating states of flicker up to a rate of only 0.5 Hz, and the limit for 15-month-olds was a rate of 1 Hz. Thus, the temporal resolution of 15-month-olds was almost 8 times coarser than the resolution observed in adults presented with the same stimuli. Despite this reduced temporal resolution of attention, the infants were able to perceive the presence of rapid flicker at a rate of 10 Hz. These findings indicate that temporal attention develops more slowly than temporal vision, and that this protracted development is specific to the selection of individual event identity in time. Furthermore, the shape of the developmental function of infants’ temporal phase individuation was markedly different from that of the developmental function of contrast detection, showing a significant effect of age only for selection of the out-of-phase flicker; thus, individuation of temporal phase relies on a mechanism independent of low-level temporal resolution. We therefore conclude that the temporal resolution of visual attention is coarse in infancy and undergoes an extended period of development beyond the 1st year of life.
This coarse temporal resolution has implications for how infants interact with, and thereby learn from, their visual environment. Temporal segregation and integration of discrete events likely play a role in infants’ eye, head, and body movements during activities ranging from anticipating the trajectory of a moving object in order to plan and execute the timing of a reach, to perceptually binding synchronous temporal events (e.g., audible speech and mouth movements) across space. In the auditory modality, fine temporal resolution is known to be especially important during language development, as the rate of individual speech sounds needs to be processed so that one can detect and discriminate phonemes, words, and sentences (Jusczyk, Rosner, Reed, & Kennedy, 1989
Converging findings from transcranial magnetic stimulation studies and studies of neurotypical adults and brain-lesion patients have shown that the temporal limit of visual attention is likely set at a high level in the visual system. These findings have led researchers to propose the existence of a “when” pathway that is localized in the right parietal lobe (Battelli et al., 2007
). The “when” visual pathway has been characterized by its functional role in temporal processing of midrange timescales (50 ms–1 s). The perception of most immediate, ongoing visual events occurs over such midrange timescales (Battelli et al., 2007
), which are considerably longer than the timescales for the localization of flicker (Holcombe, 2009
) or sound (Mauk & Buonomano, 2004
), for example, and considerably shorter than the timescales for cognitive temporal judgments, such as the experience of elapsed time.
Our findings concerning the development of temporal resolution of attention may bring researchers closer to understanding the functional development of the proposed “when” pathway and, more generally, the right parietal cortex. Little is known about when and how the parietal cortex develops, but some studies using positron emission tomography (Chiron et al., 1992
; Chugani & Phelps, 1986
) and MRI (Geidd et al., 1999
) have provided evidence that parietal areas mature substantially between the ages of 3 and 6 months (Gilmore & Johnson, 1998
), and that general changes in cortical thickness begin in the 1st year and extend into preadolescence (Greenough, Black, & Wallace, 1987
). It is therefore possible that development of parietal cortex cytoarchitecture, including synaptic and axonal pruning and myelination, contributes to more stable connections between parietal cortex and other areas involved in temporal perception.
Our results may also advance the understanding of the possible consequences of delayed or atypical development of temporal visual attention. Abnormally coarse temporal resolution early in life likely has consequences for the development of visual functions that require precise temporal sensitivity, including motion perception, attention deployment, and tracking. These processes have been reported to be impaired in multiple neurodevelopmental disorders, including fragile X syndrome, Williams syndrome, and autism (Atkinson et al., 1997
; Farzin et al., 2008
; Kaiser & Shiffrar, 2009
; Kogan et al., 2004
). Further studies are needed to investigate the relationship between temporal visual attention and the atypical development of perceptual, cognitive, and motor skills characteristic of individuals with these and other disorders.