Human beings can hear what dogs and dolphins can't

The reactions of single neurons give humans better ability to detect differences in frequencies.

By JUDY SIEGEL
cute dog 88 (photo credit: )
cute dog 88
(photo credit: )
It is common knowledge that dogs and dolphins can hear sounds that humans can't. But now Hebrew University scientists and other colleagues in Israel and abroad have demonstrated for the first time how the reactions of single neurons give humans the capability of detecting fine differences in frequencies better than animals. Their work appears in the prestigious journal Nature. The scientists did this by using a technique for recording the activity of single neurons in the auditory cortex while subjects were exposed to sound stimuli. The auditory cortex has a central role in the perception of sounds by the brain. What scientists know about the auditory cortex was largely based on earlier studies that traced neural activity in animals while they were exposed to sounds. But while such studies have supplied invaluable information regarding sound processing in the auditory system, they could not shed light on the human auditory system's own distinctive attributes. Experimental study of neural activity in the human auditory cortex has been limited until now to non-invasive techniques that gave only a crude picture of how the brain responds to sounds. But recently, investigators from HU, the University of California at Los Angeles, the Tel Aviv Sourasky Medical Center and the Weizmann Institute of Science were successful in recording activity of single neurons in the auditory cortex while the subjects were presented with auditory stimuli. They did this by utilizing an opportunity provided during an innovative and complicated clinical procedure that traces abnormal neural activity to improve the success of surgical treatment of intractable epilepsy. The main researchers were HU neurobiology Prof. Israel Nelken; UCLA and Sourasky Medical Center Prof. Itzhak Fried; and Weizmann Prof. Rafi Malach with students Roy Mukamel and Yael Bitterman. . In tests measuring response to artificial sounds, the researchers found that neurons in the human auditory cortex responded to specific frequencies with unexpected precision. Frequency differences as small as a quarter of a tone (in Western music, the smallest interval is half a tone) could be reliably detected from individual responses of single neurons. Such resolution exceeds that typically found in the auditory cortex of other mammalian species (except, perhaps, for bats, which uniquely make use of their auditory system), serving as a possible correlate of the finding that the human auditory system can discriminate between frequencies better than animals. The result suggests that the neural representation of frequency in the human brain has unique features. Interestingly, when the study participants were presented with "real-world" sounds - including dialogues, music (from the soundtrack of the 1960s movieThe Good, the Bad and the Ugly") and background noise - the neurons exhibited complex activity patterns which could not be explained based solely on the frequency selectivity of the same neurons. This phenomenon has been shown in animal studies but never before in humans. Thus it can be seen that in contrast to the artificial sounds, behaviorally relevant sounds such as speech and music engage additional, context-dependent processing mechanisms in the human auditory cortex.