The experiment described in the linked article Bazanilla & Armstrong (1972) is about a voltage clamp experiment in squid axon. Voltage clamping basically means that the potential difference across the axonal membrane can be set at will by an external artificial electronic power source.
NMDA receptors are channels that conduct positively charged ions (mainly Na+). If the voltage is clamped at negative potentials under physiological conditions, positive current will be drawn inward. This is generally plotted as a negative current, as shown in your picture in your post. If, however, the voltage is made more positive, current will reverse at a certain point, and positive currents will be measured by the electrodes. This positive current is characterized by an outward flow of positive ions through the NMDA receptor.
The sign of the current is arbitrary, but the most important point to make is that most channels allow current to pass both ways. There are rectifier channels, however, that will only allow one-way traffic. Rectifiers are more exceptions than the rule.
- Bazanilla & Armstrong, J Gen Physiol (1972); 60: 588-608