How do humans compute approximate number? According to one influential theory, approximate number representationsarise in the intraparietal sulcus and are amodal (independent of any sensory modality). Alternatively, approximate numbermay be computed initially within sensory systems. We tested for approximate number representations in the visual systemusing steady state visual evoked potentials (SSVEPs). We recorded EEG from human subjects while they viewed dotcloudspresented at 30 Hz. Alternating the dotcloud numerosity at 15 Hz evoked a 15 Hz SSVEP detectable over the occipital lobe(Oz). The SSVEP amplitude increased as the numerical difference between dotclouds increased, indicating that subjectsvisual systems were differentiating dotclouds on the basis of their numerical ratios. Critically, subjects were unable toconsciously discriminate dotcloud numerosity, indicating the rapid presentation disrupted reentrant feedback to visualcortex. Approximate number appears to be computed within the visual system, independently of higher-order areas suchas the intraparietal sulcus.

Information in speech appears in bursts. To optimize speech perception, the brain aligns these bursts of information with slow rhythms in neural excitability (<10 Hz). How does the brain track the timing of external events? Here we tested whether neural stimulus-tracking depends on participants’ expertise, or on a language-specific mechanism. We recorded electroencephalography (EEG) in participants who were experts in either ballet or in sign language, while they watched videos of ballet or sign language. We show that stimulus-tracking depends on expertise: Dancers' brain activity more closely tracked videos of dance, whereas signers' brain activity more closely tracked videos of sign language. This effect of expertise emerged at frontal channels, but not at occipital channels. These results suggest that frontal cortex forms temporal predictions based on expert knowledge. The brain may use the same predictive mechanisms to optimize perception of temporally-predictable stimuli such as speech, music, sign language, and dance.